It happens, also, out of nowhere it can be lost, usually at the beginning of the round. I've spawned in with freeday, and lost it within a few seconds without even changing class.
Alternately, there is an issue where freeday isn't revoked in some cases when taking ammo. At first, I thought it was when you grabbed ammo at the exact same moment as another freeday, but I've had cases where, especially as medic, I would grab ammo and not lose my freeday even if I was the only freeday.