Are you viewing the desktop site on a mobile phone or the mobil site?

The image preview on the mobile site predates the desktop media viewer and is not related to it in any way other than its function.

I just did some casual testing, loading the mobile site (en.m.wikipedia.org) using desktop, with preference Media Viewer off. Clicking images give unpredictable results. I get one of the following two types of result:
https://en.m.wikipedia.org/wiki/File:The_Matrix_Poster.jpg
https://en.m.wikipedia.org/wiki/The_Matrix#/media/File%3AThe_Matrix_Poster.jpg

My testing found no consistent rule to predict which I would get. Lead image / non-free images seemed more likely to give a File: link, and later / free images seemed more likely to give the media link, but there were exceptions. Repeatedly clicking the same image on a page would give the same result, except clicking a second image on a page could change the first image from file: to media. Very very strange.

I tried your link saying this was moved to Reading-Web-Backlog, but I don't see it there?

Yes that will happen. The mobile mediaviewer code is a progressive enhancement, i.e. it will try to enhance the page but not break any links, so if JavaScript fails to load (dependent on your internet connection speed, how responsive our servers, bugs in JavaScript code etc...), the normal behaviour of going to the file page will happen. All these factors are inconsistent which would explain your results, but I assure you the Media Viewer preference on the desktop site has no effect on any code that runs on the mobile en.m domain.

In T144571#2650519, @Jdlrobson wrote:

if JavaScript fails to load... going to the file page will happen

Thank you for clearing up why some of the testing gave random results. However that is not directly related to this issue.

I assure you the Media Viewer preference on the desktop site has no effect on any code that runs on the mobile en.m domain.

Exactly. That is the bug. As stated in the task description:

1. Media Viewer on m.wikipedia.org (mobile) fails to respect a preexisting explicit Media Viewer opt-out.
2. Cog icon is missing from Media Viewer on m.wikipedia.org, preventing users from toggling this preference on or off.

The mobile mediaviewer code is a progressive enhancement

Users who have actively shut the thing off clearly do not consider it to be an "enhancement".

Given the WMF's reader survey results When weighted to match global Wikipedia readership for each language, fewer respondents found the tool "useful" than "not useful" (39% "useful", 50% "not useful", and 10% "not sure")
given the community's repeated consensus that Media Viewer is worse than the file page,
given the serious long-term outrage and harm resulting from the (mis)use of Superprotect to ram out Media viewer,
the absolute minimum is to respect the opt-out setting of people have actively shut it off.

Sorry to hijack, but since this thread is coming up on google for “disable mobile media viewer”, can I ask:

Is there any currently working method to disable the media viewer on mobile, including blocking a script from loading, user style sheet, etc. I absolutely hate the thing.

Removing project MultimediaViewer since that is not used for MobileFrontend.