My player is a rigidbody cube with a box collider that can push other "NPC" rigidbody cubes that also have a box collider. These NPC cubes have restricted movement either in horizontal or vertical direction. If the player pushes a cube that can only move horizontally into a cube that can only move vertically, the cubes start clipping. I recorded the whole situation in this video. Note that the NPC cube on the left has frozen X position, while the one on the right has frozen Z position.
I know that one way of preventing this would be to increase the mass of the NPC cubes, but that would only resolve the problem proportionally to the increased mass (the higher the mass, the lower the clipping), and at some point my player is not strong enough to move the NPC cubes anymore.
How can I solve this?