I made a cuboid and moved it into position. As soon as it gets into position, one side fragments into a number of strange pieces. I tried deleting them and drawing a new side (just one rectangle). That fragments exactly the same. There are no visible lines - it can only be seen when you click on the pieces. It wouldn’t be a problem but I’m going to put a moulding on and i can just see this being a problem.
I’ve push-pulled the fragments for illustrative purposes.