This is a tricky one! I see where you’re coming from @NateDhaliwal but I don’t think this is a bug.
Watching a tag is a shortcut for watching topics. While the user had access to the tag, any topics created with it were automatically marked as watched. I don’t necessarily think it’s a bug that those topics remain watched just because they stopped watching the tag (by virtue of it being removed from their access).
But let me check with @hugh, our staff experience PM, to see if he agrees. I could probably argue in either direction but I think this likely should be considered a Feature request, and could cover broader tracking changes rather than just changes brought on by losing access to the tag.