To be honest, I don’t have any experience with deaf dogs. But thinking logically - the vestibular system is also very much connected to hearing… Maybe being upside down just makes your pup uncomfortable because of his deafness. Don’t know.
Anyways, I guess that the fact that your frenchie doesn’t sleep on his back isn’t really something you should worry about, as long as he is healthy and happy. After all not all of us find same positions comfortable. (I personally never sleep on my back either, LOL)