If you add a link to a user page using the link dialog, the preview link in the link dialog opens a new window with "user:Foo" rather than the full https://..../wiki/User:Foo
Inside ve in general, on an article page. I didn't try in Flow. I added info to a mediawiki page about the hackathon and used the link inspector to try and link my mentee's user page, when I saw the issue.
Firstly, this is a preview glitch only, as the wikitext when you save the article works exactly as you'd expect. Secondly, adding a link to a user page in an article is incredibly rare. In a quick search I found only two articles on the English Wikipedia with links to user pages. Based on these factors, this is not high priority.
Same problem with Flow.
I have not achieved to reproduce with user: prefix, however it happens with help:, meta: and template: prefixes, so it affects all meta page (help pages, template pages, all metawiki and mediawiki pages...).
Note that it affects template: prefix on metawiki but not equivalent modèle: prefix on fr.wp.