Yes, they might do that. My LO does it all the time.
But there are safety mechinisms in their bodies to address that. If they start breathing in too much CO2 because their noses are smooshed, the CO2 levels go up in their blood stream. This reaches the brain... and the brain says... OH NO! The brain then triggers them to move their heads until they breathe better. That is why it is generally safe once they have good head and neck control
The dangers come when they aren't able to move their heads well (as they are too young and not strong) and even though the brain says MOVE the muscles can't manage it.
The only other danger is if, for some reason, that mechinism is faulty (like a congential abnormality). That is VERY VERY VERY VERY rare! And there is no way to test for that problem.
But it is so rare that it is highly unlikely your LO has this problem. And it will have to be tested at some point! You can make him sleep on his back forever!
So, chances are, your LO will be just fine if he decides to roll to his tummy!