Shoot, I did some further testing and the game must be considering the stun ticks a turn. That's a problem, I'll have it fixed ASAP.
EDIT: Found a workaround! Instead of using the built-in "Cannot Move" function of RPG Maker, I just baked the stun effect into the speed formula. If I need to, I can use that for any later stun effects! Will have the patch up ASAP.