I might have figured out a workaround here. It’s painful, especially if you have a lot of marketplace 2020 content but it seems to have worked for me. PC only unfortunately. Feedback appreciated.
Exit out of MSFS2024 completely.
Find %AppData%\Local\Packages\Microsoft.Limitless_8wekyb3d8bbwe\LocalState\StreamedPackages
In that directory find the folder name of the broken whatever
Create an empty folder in the 2024 community folder of that name. This folder needs to have the same name but needs to be empty.
Launch sim and see if you have defaults or still the broken 2020 package.