Hi, I had the exact same problem, after much confusion scratching of heads it seems since the Xbox version came live something changed within the Microsoft store, i owned the boxed version- FIX re install WIN10 , install boxed version of MSFS2020, first run it will take you to the store where it will proceed to install gaming services and patch the the sim after clicking the INSTALL! button , fire up MSFS 2020 it will 89GB down load to the current latest patch . It seems it will install to the default directories this time around and it would not install into the game drive B in my case . The app I found would not move to B drive either ,once I had a fully working patched flight sim however I created a symbolic link for the community folder into B drive . All in all 12 hours of work . ( I purchased a 1TB 980 pro for clean install of Windows/Flight Sim as the repair on the old drive was impossible.
I hope this helps bad news I know it was the only work around.