I’d like to let you know that the problem still persists in 2.8.0.beta4 (Commits · discourse/discourse · GitHub)
We haven’t updated yet to the latest commit as there are no changes present to fix this bug.
In our case, user flairs remain unchanged when we update groups via Discourse Connect SSO. We see the user is no present in the group, but flair remains.
Hope it helps identifying and fixing the bug.