Please select a lower cruise altitude if the aircraft is struggling to maintain the target airspeed. The slower you fly the less thrust is available at these high mach numbers. So staying ahead of the drag curve is important. Try a lower cruise level like FL 370. If you're fully loaded you may have to stay down as low as FL310.
The transition altitude and level being ignored is a bug which I was able to fix on my side.
It is off topic but were you able to fix that when doing performance calculations on 777 and 787 the weight was ignored? Like when pressing compute it came back to a default weight.
Thanks!