The "bug" is twofold.
First is that air over 60k is like flying at sea level.
The second is that the c47's engines are bugged and instead of losing power, can run at an incredibly high MP. (once over exactly 27000feet, the c47 burns fuel unbelievably fast, as it puts out an incredible amoutn of power) It will rip itself apart at full throttle in level flight even.