The only circumstances I'm aware of where you do not receive an event is if someone else was assigned as Operator instead of you. It could have been done by the station Owner or Manager. The event is only sent when the Unassign option is used, not if you are replaced by someone else with the Assign option.
Stations do not simply unassign themselves from people on their own.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."