Are you using Xbox friends & family sharing? To my knowledge, this doesn’t work with MSFS.
There is an FAQ here:
https://flightsimulator.zendesk.com/hc/en-us/articles/4755091663900-Family-sharing#:~:text=There%20is%20no%20license%20sharing,account%20that%20purchased%20the%20sim.
If you are not using friends & family sharing, then assure that both accounts on the device are signed into their own account used to purchase MSFS.