I 'm currently farming Westbridge Hills and that's where I noticed the problem. Tonight I'll try it on a different save game and see if the problem is present but I seem to remember having the problem on Bjornholm too and so not using it there.
Edit: I played tonight and could not reproduce the problem on any save or map. Last night was consistantly happening. I could sell the Puma back and buy a different tractor and problem would dissappear. Buy the Puma and it would reappear. I tried it several times always with same result. I'm of the opinion now that perhaps it was just a problem with that gaming session.
Playing on PC