The mobile diff comes from wikidiff2 diff code lives inside core.

My $0.02: the diff on mobile looks like it's the one behaving correctly here, as the page exists.

In T294690#7471902, @Jdlrobson wrote:

The mobile diff comes from wikidiff2 diff code lives inside core.

The diff body, probably not the edit summary though?

My $0.02: the diff on mobile looks like it's the one behaving correctly here, as the page exists.

The page doesn't exist, but is known (in the sense of Title::isKnown()). So this is probably a matter of using that vs. Title::exists().
Agreed that the mobile behavior is the correct one. Known pages should be blue in edit summaries, just like in the article body.

Ah right. I hadn't realized that was the summary. The styling was too similar to the diff I thought it was part of the page. Thanks for correcting me @Tgr

Sounds like this should be a relatively straightforward fix, so prioritising this while we're blocked on some other RecentChanges work.

I was having trouble reproducing this locally, which led me down an interwiki config rabbit hole. So I went to double check on enwiki (where I probably should have started) and I'm not reproducing it there either. My user page only exists on meta, all others are copies from that source. I added a links to my user page on enwiki and commons and they seem to render fine. Perhaps this issue has been resolved since it was last looked at in 2021?
https://en.wikipedia.org/w/index.php?title=User%3AJSherman+%28WMF%29%2Fsandbox&date-range-to=2025-03-21&tagfilter=&action=history

Could you verify @Edu?

Just confirming that I also can't reproduce this error - I tried on a couple of wikis, including one I had never logged into locally before - apologies for not double checking this earlier!