None of those planes are technically capable of flying at those altitudes to begin with. Just because you can doesn’t mean you should. The issues you experience are fairly consistent when flying above your limits. Engines are only capable of sucking up so much oxygen and wings can only provide so much lift before the conditions. Let me ask you a question, whats your angle of attack in these planes at FL445 (the angle of aircraft pitch in relation to the horizon)?
However I can see it being an issue once a Concorde has been released since that generally travels at FL600. Does the 747 have this issue since it’s the only plane in the game that’s actually capable of reaching those altitudes?