I confirm the problem.
OS: Windows 8.1 x64.
Browser: Mozilla Firefox 115.3.1esr x64.
Translated article: https://ru.wikipedia.org/wiki/%D0%9A%D0%BE%D0%BD%D1%82%D0%B0%D0%BA%D1%82
The preserved article: https://uk.wikipedia.org/w/index.php?title=%D0%A1%D0%BF%D0%B5%D1%86%D1%96%D0%B0%D0%BB%D1%8C%D0%BD%D0%B0:%D0%9F%D0%B5%D1%80%D0%B5%D0%BA%D0%BB%D0%B0%D0%B4_%D0%B2%D0%BC%D1%96%D1%81%D1%82%D1%83&campaign=contributionsmenu&from=ru&page=%D0%9A%D0%BE%D0%BD%D1%82%D0%B0%D0%BA%D1%82%D1%8B+Windows&targettitle=%D0%9A%D0%BE%D0%BD%D1%82%D0%B0%D0%BA%D1%82%D0%B8+Windows&to=uk&revision=124585601 (the article was translated in its entirety)
Screenshot: http://prntscr.com/-MHQTkpPbye9
My post on MediaWiki: https://www.mediawiki.org/w/index.php?title=Topic:Xrdi08uqph6m8y6g&topic_showPostId=xrmi0b2swfwhym0z#flow-post-xrmi0b2swfwhym0z

After trying to complete the translation, it stopped loading altogether: http://prntscr.com/tVk23A2ET_t_
URL: https://uk.wikipedia.org/w/index.php?title=%D0%A1%D0%BF%D0%B5%D1%86%D1%96%D0%B0%D0%BB%D1%8C%D0%BD%D0%B0:%D0%9F%D0%B5%D1%80%D0%B5%D0%BA%D0%BB%D0%B0%D0%B4_%D0%B2%D0%BC%D1%96%D1%81%D1%82%D1%83&campaign=contributionsmenu&from=ru&page=%D0%9A%D0%BE%D0%BD%D1%82%D0%B0%D0%BA%D1%82%D1%8B+Windows&targettitle=%D0%9A%D0%BE%D0%BD%D1%82%D0%B0%D0%BA%D1%82%D0%B8+Windows&to=uk

I have encountered the same problem when translating sv:Mavia to nnwiki and en:Dick Gaughan to nnwiki. When I try to load the saved versions of these translations, I get the error message "Unable to fetch draft information." The first was last saved 7 days ago, the second was saved 5 days ago. Later translations (working from nowiki) have not had this issue. However, it's continually being reported from various users trying to translate articles.

At pt.wiki we have the same problem, translating from any language to our portuguese language. As of now, the problem continues.

In T348797#9279818, @Ranveig wrote:

I have encountered the same problem when translating sv:Mavia to nnwiki and en:Dick Gaughan to nnwiki. When I try to load the saved versions of these translations, I get the error message "Unable to fetch draft information." The first was last saved 7 days ago, the second was saved 5 days ago. Later translations (working from nowiki) have not had this issue. However, it's continually being reported from various users trying to translate articles.

This issue appears to be a different issue than the one in the task description, which reports that "Some parts of the article translated disappears (did not save) from the translation after publishing.". The issue @Ranveig reports is caused by a known bug (T349311) that will be fixed in production wikis within this week.

In T348797#9282251, @Tuga1143 wrote:

At pt.wiki we have the same problem, translating from any language to our portuguese language. As of now, the problem continues.

@Tuga1143 it is unclear if the problem you are mentioning is the same with the issue Ranveig mentions, or the one mentioned in the task description. Could you please clarify which one you experience?

No, the problem has not disappeared.

In T348797#9290855, @ngkountas wrote:

In T348797#9282251, @Tuga1143 wrote:

At pt.wiki we have the same problem, translating from any language to our portuguese language. As of now, the problem continues.

@Tuga1143 it is unclear if the problem you are mentioning is the same with the issue Ranveig mentions, or the one mentioned in the task description. Could you please clarify which one you experience?

Hi @ngkountas . Sorry for the late reply. At pt.wiki, the problem is gone. Now, when we open a saved translation, it shows the previous text we had translated before.

@Pginer-WMF following the above confirmations I believe that we can close this issue as resolved. Moving this to "Done" column.

Hello @MMH-fabr! Sorry for closing this ticket without all concerns being addressed. Unfortunately, we would need more information to try to reproduce this issue, as it's now a known issue. I'd like to inform you that sharing the translation URL is not helpful as the contents of the page are personalized based on the logged-in user, and thus we cannot see what you see.

Trying to understand how this situation occurs, I'll ask some questions:

• Did you translated a lot of sections/paragraphs in a row by clicking on the placeholder section? Did you edited any or all of the translated sections?
• Could you please share any related errors that appear in the developers console?
• Would it be possible to share the result of the latest autosave request (the one that froze) by checking the Network tab of your Developer Tools? The autosave requests are POST requests to the "api.php" endpoint that include the "action" parameter with "cxsave" value, like the one in the screenshot below

• Finally, could you try to find your translation in our debugging tool (https://cxdebugger.toolforge.org/) and check if the contents you translated are present in the second tab, named "Translation"?

Thank you for your help and your patience!

"Did you translated a lot of sections/paragraphs in a row by clicking on the placeholder section?" - yes.

"Did you edited any or all of the translated sections?" - any.

"Could you please share any related errors that appear in the developers console?" - before continuing the translation, there were the following errors: http://prntscr.com/g-qQJM0AtaFj I continued the translation and now the errors are: http://prntscr.com/zTls92KXKkpm

"Would it be possible to share the result of the latest autosave request (the one that froze) by checking the Network tab of your Developer Tools? The autosave requests are POST requests to the "api.php" endpoint that include the "action" parameter with "cxsave" value, like the one in the screenshot below" - after the autosave freezes:

1. console: http://prntscr.com/tB_qDbfyv8xc
2. network: http://prntscr.com/GlScct4WDbx0

"Finally, could you try to find your translation in our debugging tool (https://cxdebugger.toolforge.org/) and check if the contents you translated are present in the second tab, named "Translation"?" - I found - a part of the translation that has not been preserved is also missing there.

A screenshot of the translation autosave hanging: http://prntscr.com/urRIu1qVMuTi

Here's another screenshot of the errors and warnings after (another) translation autosave freezes: http://prntscr.com/d-LVtSfarqoB

Thank you for all the information you provided @MMH-fabr! I invested a lot of effort but still I wasn't able to detect the root cause of the issue. For now we'll improve our error logging system, so that we can better catch such errors and we'll appreciate any new information about it in the future. I would suggest that you wait for the machine translation for a section to be loaded before trying to translate the next one, I have found that this mitigates the issue. One last question: have you noticed that the issue is happening for all translations or it only happens when translating some articles that meet specific criteria?

Change 973322 had a related patch set uploaded (by Nik Gkountas; author: Nik Gkountas):

[mediawiki/extensions/ContentTranslation@master] CX save failure: Improve error logging

https://gerrit.wikimedia.org/r/973322

In T348797#9322204, @ngkountas wrote:

One last question: have you noticed that the issue is happening for all translations or it only happens when translating some articles that meet specific criteria?

I managed to translate several articles without getting this error, even before it was reported that there were fixes that were supposed to fix the error. But these articles did not differ in anything special.

Change 973322 merged by jenkins-bot:

[mediawiki/extensions/ContentTranslation@master] CX save failure: Improve error logging

https://gerrit.wikimedia.org/r/973322

A while ago I reported (T346653) that translations are not being saved once the source page had changed, but there were notifications of something strange happening.
Now I'm observing the problem of this ticket while translating en:Photosynthesis, which has not been changed in the last 10 days.

I hope someone will take these reports seriously. In its present state, the CT tool is unusable for any articles longer than a page. It keeps reporting that the changes have been saved, but once you re-open the translation, they are no longer there.

I have encountered this problem before when translating from English to Arabic.

Verify that the content translation tool is compatible with the browser being used for translation. Browser-specific issues, such as differences in behavior or compatibility, may contribute to the problem.
Analyze any error messages, console logs, or other indicators that might provide insights into why the auto-saving stops. This step is crucial for identifying the root cause of the issue.

@Osps7 Unfortunately, this might be impossible for any users to debug. A developer should start translating a (long) article, leave it open over a day or two, monitor what's happening from both browser and server side. Close the browser, re-open the translation, remove wikilinks, edit the wording, etc.
For me, the tool was reporting it was saving the changes for over an hour. Two days later they were gone. I don't really know how to trigger this behavior, and trying to figure it out while doing anything useful might be a huge waste of time (as it was several times by now). The browser console is full of errors, of course, but it's hard to tell which ones were relevant over the course of a few days.

My browser is Firefox 120 64-bit on Linux.