OK so this happened a while ago and after posting about it on the steam forums and getting 1 reply saying "cool" I forgot about it. Thought it might be worth posting here.
While duo with a friend he dropped out of the plane and half the walls were missing for him. We worked out that it was only walls that had windows on them and that if he walked up to where the window was supposed to be he could prone and them move through the wall unimpeded. He also could prone and clip through furniture and then stand up inside the furniture. I dont have screenshots from his end but I do have screenshots of him standing inside a tv.
No idea about what caused it.