I was unfortunately having the same issue with a new test actor with the same logic, but I did find the solution (which was a bit ridiculous).
It seems as though the gun I was holding was somehow colliding with the vehicle and producing these odd physics. The guns are supposed to no-collide when picked up, but for some reason weren’t (on the client specifically it seems). I disabled their collision with vehicles specifically and problem solved 
So yeah, nothing wrong on Unreal’s end, my fault. I just couldn’t figure out why for a while.