Hi russoo , thank you for your feedback and providing your project. It's actually a known issue we are working on.
If you create a cube the faces of the sides automatically gets defined. One as outside, one as inside.
So if you mirror them this way, the outsides are in the inside of the cube. You can check this by walking into the cubes. You will see all sides inside the mirrored cube and the no sides inside the original cube.
