If a tag is in a tag group that is dedicated to Tags are visible to everyone, but only staff can use them it can’t be selected in User preferences for default tags watching
That is bad because we have a tag called “Announcement” and only staff can tag posts as an announcement, but we can’t set it to “watched.”
A workaround is to first create the tag and doesn’t have it in a restricted tag group, add it to default tags watching, then assign it to the restricted tag group.
But it should not be this way IMHO.