If you set up a global default tag in Stream Info, the stream goes live with those tags on it, and then you remove one of those tags during the stream, it will come back and be locked once the stream becomes a video. Since it's locked, it can't then be removed again (at least by the Editor role).

Ideally, any tag removed during a stream will not be put back for the VOD but, if that behaviour is intentional, they should at least not be locked. The particular tag in question is the Family Friendly one, & that should probably never be locked anywhere on the platform for obvious reasons.