Since the changes to vehicle remote control from the commander/gunner seat in 1.78, the AI cannot mount or dismount any vehicle that is being remotely controlled by a player from any commanding seat, when the same is stopped.
When being told to mount the vehicle, they simply walk to a certain distance from the entry point and stop.
When being told to dismount, they simply stay in the vehicle.
In either case, moving the vehicle forwards or backwards by any amount will allow the AI to complete the order. Removing the driver from the vehicle (via a script), switching to the driver seat (so it's under direct player control), getting out of the vehicle (so it's not commanded by the player anymore), or otherwise giving the control of the vehicle back to the AI (by telling them to stop manually, via 1-6) will also allow the AI to carry out the order.
Again, this only happens if the player controls the vehicle remotely from the commander or gunner seat (via the WASD keys). Telling the AI to move somewhere via the command menu, driving the vehicle manually, etc, don't cause the bug.