@Aklapper, perhaps this is only reproducible with non reviewed delta. The link for reproduction, I have posted above, it originally lead to unaccepted changes. Meanwhile since the creation of the ticket they have gotten reviewed though and merged. I should have preserved the markup of the page.

I checked it on https://pl.wikipedia.org/w/index.php?title=Postaw_na_milion&oldid=68850374&diff=cur (pending change at the time I checked), and it correctly points to https://pl.wikipedia.org/wiki/Specjalna:Wkład/2A02:2454:901C:1B00:F967:F787:E204:EE81.

I cannot spot it either anymore. I am fine with closing the report as Invalid. I will keep an eye on this and file a new one / reopen (not sure what's the desired process here) once I observe repro.

BTW where do I find this extension source code?

In T317482#8433177, @Smyru wrote:

BTW where do I find this extension source code?

The source code is in git, you can use your browser under https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/FlaggedRevs/+/refs/heads/master to browse through the source files or use a git copy, see https://www.mediawiki.org/wiki/Gerrit/Tutorial#What_is_Git? for more information (The tutorial is to change things, if you want only have a look the information on that page could be a bit to much, but I have not see a help page to just look at the source).

OK, I have stumbled upon another reproduction eventually. Please observe this set of unrevised changes by an anonymous IPv6 user: https://pl.wikipedia.org/w/index.php?title=J%C4%99zyk_portugalski&oldid=69053199&diff=cur&diffmode=source

The field affected is:

<fieldset class="portlet pBody diffhistorybox" …>

The class diffhistorybox is provided by https://pl.wikipedia.org/wiki/MediaWiki:Gadget-diffhistory.js and not visible for logged-out users, making the steps to reproduce not helpful here.
As this is a problem with a gadget please ask the local community.

There is regex in the js which only works for IPv4 and could be the problem described here (was added with https://pl.wikipedia.org/w/index.php?title=MediaWiki:Gadget-diffhistory.js&diff=prev&oldid=27358446). Maybe use of function mw.util.isIPAddress instead can help here

That's not directly obvious what code generates various parts of the view. Thanks for explanation, I will continue work on this in the local community.