I have the tracker 'user' field set to show only 2 groups - in my case, #4 is 'Field Operations', and #10 'ZZR':
All are in #4, but when an employee leaves, I remove them from this group. Yet when I use the 'user' field in TrackerFilter, the dropdown still shows even those that have been un-assigned from this group (and I've cleared the cache and re-indexed a million times since the employees left). These ex-employees are left as 'registered', or group #2, which should not show in the dropdown.
Could this be due to a misconfigured server and SSL certificate? As I am constantly experiencing odd problems.