Thanks, got it.
Confirmed, the “watched” status on topics that became watched solely because a user was watching a tag remains, even if the user later loses access to that tag after tag group permissions change. They’ll continue to receive notifications on those already “auto-watched” topics, but not on new topics created after the change.
I can see the case for treating this as expected behaviour rather than a bug, but I also see the downside of leaving hundreds of topics in a watched state. I’ll leave it to the broader team to decide how we want to handle this.