I know this sounds dumb (you'd think if the thing is dead, shouldn't the buff be over?) but I could of sworn I saw that described somewhere, even if an entity was destroyed periodic buffs could take up memory...
Well I have to double check, but I think I have buffs that have finishing conditions that use "First/LastSpawnerNoLongerHasBuff" to buffs that have no finishing conditions. Naturally I only want these to end on the target death, and I think they work as intended. Granted I suppose the buff could still exist in dead virtual space somewhere if the check was just on the target, not the buff, but that would be an odd way to do it.