Tickets can be watched by users who are not available in specific tracker
It's basically the same as solved here: #23172 - but now it's not about assignee but about watchers.
In 3.3.1 assignee list is fixed, so now can be issues assigned only to users available in selected tracker.
The same shoud be fixed for watchers. Now we can add watchers which are not available in specific tracker - the list of possible watchers shoul be limited only to users available in selected tracker.