Whenever a mission is launched, and the Headless Client is already on the server, it automatically jumps into a slot (seems it is the first slot in the mission.sqm?).
That slot disappears from the slotting screen, even if the HC is manually moved to another slot, and cannot be accessed anymore.
Moreover, ForceHeadlessClient = 1; doesn't work: the slot with this line is greyed out in the slotting screen and nobody can access it. {F22226}