It works for me. I am unable to replicate it.
Can you provide a specific test-case?

kunalgrover05: Thanks for testing this! Could you please describe exact steps how you tested this? URL, plus what you did?

Tested on a local wiki
1- Translate a page to a language.
2- Select review mode
3- 'Hide own translation' hides all my translations
4- Login as a separate user.
5- 'Hide own translation' doesn't hide other user's translations in review mode.

Things I would also test:

• Make some translations first in different modes of list/page/review
• Scroll down to see more messages with own messages hidden or shown initially and try toggling the state again.

Purodha: Can you still reproduce this? Which exact Firefox version is this (as "latest" changes every few weeks and depends on your update policy channel)?

If this still happens, could you please open the Error Console (or whatever it is called in your browser) and reload the page that you see the problem on?
If there is a problem or an error with JavaScript it should be printed in that console.
For more information: https://developer.mozilla.org/en-US/docs/Tools/Web_Console

I'm unable to reproduce. Please note that the default view (unreviewed) of review mode does not include your own translations in the first place, so there is nothing to show hide.

To make this behavior clearer, the button should be hidden when there are nothing to hide. I updated the summary to match that.

Change 213248 had a related patch set uploaded (by Nemo bis):
Don't offer to hide own translations when already hidden in proofread mode

https://gerrit.wikimedia.org/r/213248

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I am waiting on review of https://gerrit.wikimedia.org/r/304828 so that this patch is easier to make work properly.

This is a relatively small task from the Translate backlog.

Change 213248 merged by jenkins-bot:
Don't offer to hide own translations when already hidden in proofread mode

https://gerrit.wikimedia.org/r/213248

I tried to deploy this to translatewiki.net, but due to T146873 I had to rollback. @Amire80 You can test this on dev.translatewiki.net if you don't want to wait for that bug to be resolved.