Taking someone out of class and they return

I guess the reason is that once you been admitted into the meeting you will be able to bypass the meeting next time is only because make it easier for attendees and presenters, could be distracting if someone leaves and rejoin the meeting many times.

But I see the problem and there is a uservoice requesting that if you rejoin the meeting you should be put in the lobby second time too.

https://microsoftteams.uservoice.com/forums/555103-public/suggestions/39481747-issue-with-guests-reentering-teams-meetings-and-by