Description:
Any locomotive which is pushing other carts will change direction if the carts get stuck or if they move slowly.

How to recreate:

1. Place track down, make sure there are a couple of curves in the middle (not sharp curves but slightly)
2. Place a locomotive and some minecarts down
3. Link the carts and locomotive together, and make sure the locomotive will be pushing the carts, not pulling
4. Turn the locomotive on and change the settings so it can push it
5. The locomotive will struggle to push the carts on the curved section of track and the locomotive would change direction without reversing/un-reversing it (with the button)

This bug can be repeated even when the locomotive is moving forward in the direction of movement through several smoothed or sharp turning angles, regardless of whether there is movement in the traction or on the run-out (run-out is movement without the use of traction due to the guiding slope). When pushing a train of 20-30 trolleys, such a bug can be achieved when moving along a straight section of the track, but with an upward movement along an inclined path.

I can record a video of a real problem in confirmation.