Nautopilot does not provide traction power sometime

I designed an automated ferry that would allow ships to cross while the player could easily cross, and it initially worked well. But after the server was saved and restarted a few times, something strange happened.

Normally and logical, two nautopilot systems heading in opposite directions would balance, keeping the boat stationary. The addition of a third nautopilot would tip the balance and send the boat heading in one direction.

As shown in the video, the first boat is normal, with two nautopilot providing more traction than one, and the boat moves. The second boat is affected by bug, and the third nautopilot was turned on, providing no traction and the boat remained stationary.