Originally Posted by: Peregrine57 
I've encountered this in previous Forza games too, just this weekend. I don't remember which one, because i touched on multiple, but I think it was Horizon 3. I was trying to get a car into a particular class for Rivals, and I reset the rims to stock, and it knocked my PI way down. It took me a while to realize that resetting your rims resets your tire width as well. The fact that I've seen this across previous games makes me think it's not necessarily a bug, but maybe by design.
Did you watch the video clip above? That can only be a bug, surely.