It’s working as designed, so one of those edge cases between it being a bug, because one might expect the username to update automatically, and a feature request.
There is an open issue for this on github, which usually means the staff would like to implement this, but it’s limited by their time. I’ll close this one though since there is already an existing topic for this: https://forum.inaturalist.org/t/make-mentions-unique-to-user-id-not-just-username/9041 (feel free to weigh in there)