Original report: https://github.com/mattermost/mattermost-server/issues/11903
This is not a regression.
Following an invite link while logged in as another user invalidates the link but doesn't allow the new user to sign up.
Steps to reproduce
1. Enable email invitations (System Console > Authentication > Signup > Enable Email Invitations).
2. Invite someone by email (switch to team, Burger Menu > Send Email Invite) (e.g. invite yourself via another email not yet known to Mattermost).
3. Click on the link in the invitation email while logged in as another user.
Either it should be possible to sign up as the new user despite already being logged in; or, if not, there should be a message saying that signup only works when you're not logged in; most importantly, in the latter case, the invitation link shouldn't be invalidated by this failed attempt.
The link doesn't lead to the signup page but to the normal user interface for the user who's already logged in; there's no indication of a failed signup attempt. If you now log out and try to use the invitation link again, it doesn't work anymore (though this only becomes apparent after you've filled in the form and try to submit it). A look at the database confirms that the invitation token is deleted on the first attempt.