Endless momentum/ downhill force

Plattform PC, keyboard + mouse, Win 10 64bit

I came along to a bug I wasn't able to reconstruct on purpose, it happened two times with different vehicle and trailer:

I was driving the K700 + short logs trailer on proving ground and at a very muddy area, I stopped- but the vehicle didn't.
There was a slope so it was expected that the vehicle would move on a little bit, forced by the momentum, but even with deep mud and at the end of the slope it was not stopping, it was moving on. I put in neutral gear but it did not stop, handbrakes didn't work either, the vehicle and the trailer moved on. The brakes did work and stopped the vehicle but when released, it starts moving again O.o'
After crossing the whole muddy area on proving ground, there was a upwards slope, the K700 drove an it, stopped and started moving backwards...

The second time it happened with a C432011 + medium logs cart on the map level_river, same situation, the ground was very muddy and the vehicle (unloaded) started to drive uncontrollable.

To me it seems to be a problem with the downhill force and the momentum as there must have been a force that pulled/ pushed the vehicle nonstop.
Changing the vehicle didn't help. After I closed and resumed the map, the bug was gone. The bug seems to be very rare.

if you have a controller plugged in (on PC), it will do this sometimes. the controller could have a slightly stuck button. try smacking the triggers and see if it stops. also, turn the game off, unplug then plugin the controller, and restart the game. controller could also need calibration. you can also unplug the controller in the middle of this and use the keyboard thus deactivating the controller fixing the issue.

I have a Steam Controller but it is turned off. And if it would be a problem with the controller, the problem would be there all the time.
But it only happens when a vehicle stands or drives through mud and if there is a slope. It does NOT matter if there is a trailer attatched or not, I was driving a Zil130 without trailer and it startet to move without any input.

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