Seems to still be happening as of MediaWiki 1.44.0-wmf.25 (964d7fd). The HTML source of (to take an example from Special:Random) https://test2.wikipedia.org/w/index.php?title=14thmay&diff=prev&oldid=157204 contains the incorrect tag <link rel="canonical" href="https://test2.wikipedia.org/wiki/14thmay">.

Was I subscribed here inadvertently?

Oops, sorry about that.

I hope that’s the right place for it (as the description mentions HTML rendering) - I thought it might be more useful to at least try to put it somewhere appropriate rather than moving it back to General (with a non-zero chance of being correct, and equal effort to fix if it’s incorrect)

@Aklapper Sorry, regarding the project tags - now that I’m looking at it the next day the issue is broader than the title suggests (as described in the body). The code in question looks like it happens for many (all?) page renders (as opposed to being part of the diff functionality), and diffs aren’t the only type of page where that canonical rel is inserted inappropriately.