I had the same problem. The Microsoft team are working on solving it.
But there is a work arround. In:
C:\Users\your name\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\MISSIONS\Custom\CustomFlight
Delete the file CUSTOMFLIGHT.FLT (just leve the otheres) before you start the simulator
You have to do it every time you start from the globe-menu.
Stay safe!
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