Ok, I suspected this for a while, but there are definitely cases where officers are "released from service" as "surplus to requirements" when they most certainly HAVE been given assignments recently.
Just a moment ago, I reassigned my Fleet Headquarters commander, since an officer had been promoted that had a higher Crew Training score. The very next turn, the guy who was just relieved (and had been the fleet commander for a while) was released as not having had an assignment in 12.4 years.
So, basically, is any officer in danger of being "released" if he isn't CURRENTLY assigned? Or are only certain things (staff? teams?) not counted as "assignments" for job history purposes? Either way, it's clearly a bug.