Ok interesting... I know that there is a delay of about 2 seconds after jumping off a vehicle that SA-MP still actually detects that you're on the vehicle. Kye calls this the 'stick time' or some shit. It's to help people stay on if you go over a bump or something.
I don't really know how much I can do about this tbh, lag and accuracy would make it impossible to add a foolproof check. But I'll look into it.