This happens in both P-39D and P-39Q, and happens with the 20mm cannon as well as the 37mm.
Goto external view on the runway and swing your view to look at the propeller hub cannon from a front-quarter location. When you fire the cannon, if a propeller blade is in your line-of-sight, the muzzle flash occurs behind the propeller blade. If you adjust your view slightly so the blade is not in your direct line of sight behind the cannon muzzle, the muzzle flash will appear normally.
I think this is called a graphic sorting error.
btw I just learned this today, but the P-39's cannon muzzle has quite a visible recoil:
