I did a little bit of investigating to try and find out why - when a user has Advanced Mobile Contributions enabled - having FlaggedRevs active on a page causes the "(talk | contribs)" links to disappear (and an icon next to the editor's name to appear) when the screen width is <640px.

From what I can see in my browser, the CSS statement causing "(talk | contribs)" to not be displayed is one that targets .mw-usertoollinks, with the rule display: none !important;. Looking through this codesearch, the only fully matching CSS rule I could find was this block within MinervaNeue's skinStyles/mediawiki.diff.styles.less.
Another codesearch for what might be loading mediawiki.diff.styles when viewing a page's history found this line in FlaggedRevs' extension.json that declares it as a dependency for ext.flaggedRevs.advanced - a module that appears to be loaded (among other places) by a function called by the BeforePageDisplay hook.

My working theory is therefore that FlaggedRevs seems to be loading MediaWiki diff styles when an end user views the history of a page that has FlaggedRevs enabled; which in turn is loading Minerva's skinStyles/mediawiki.diff.styles.less, which in turn is causing the "(talk | contribs)" links to not be displayed & an icon to appear next to the editor's name (and potentially also some other issues, though I haven't looked into anything else).
I've therefore tagged MinervaNeue (Tracking) on this task, as - unless I've completely messed up (which is always possible) - this seems to be a side effect of Minerva stylesheets being loaded where they shouldn't be.

(as Advanced Mobile Contributions's been archived)

While MobileFrontend provides the mobile domain, the codebase here has nothing to do with the bug.

The module mediawiki.diff.styles should only be loaded to style diffs but I don't see a diff on https://en.m.wikipedia.org/w/index.php?title=Square&action=history
What am I missing? If no diff is being rendered the module mediawiki.diff.styles should be removed.

As a short term fix .mw-usertoollinks could be scoped to the .diff selector e.g. .diff .mw-usertoollinks.

In T387091#10766609, @Jdlrobson-WMF wrote:

While MobileFrontend provides the mobile domain, the codebase here has nothing to do with the bug.

Fair enough - I was probably assuming that (as an issue affecting AMC, which I believe is provided by MobileFrontend) it would go under MobileFrontend (Tracking) if nothing else, but apologies if I got this wrong/should have just added it to the 'Tracking' column myself.