Also reproducible on beta cluster: https://commons.wikimedia.beta.wmflabs.org/w/index.php?title=User:AJ/Sandbox&diff=257063&oldid=257062

But it doesn't always happen, https://commons.wikimedia.beta.wmflabs.org/w/index.php?title=User:AJ/Sandbox&diff=257061&oldid=236143 shows a blue link. But when Nardog linked my userpage on enwiki it was red, so it's not limited to linking your own user page.

Still an issue.

https://en.wikipedia.org/w/index.php?title=Wikipedia:Files_for_discussion/2021_December_31&oldid=prev&diff=1064318408

Also happening on https://www.wikidata.org/w/index.php?title=Wikidata:Report_a_technical_problem&action=history

Might be a duplicate of T294027: Username appearing as redlink in history, even when page exists locally.

(Also tagging watchlist because it happens there as well.)

Also in edit summaries on diff pages per T293665: User page link is red in diff and history pages edit summary despite global user page existing.

Often this is due to code using Title::exists() (which checks whether the title physically exists in the page table) instead of Title::isKnown() (which accounts for things like special pages, and
GlobalUserPage can hook into it).

The relevant component is CommentFormatter, which I think has no Phabricator tag or Maintainers entry, but the most closely related one is probably the parser.