I uninstalled everything — base sim included — and wiped every temporary memory outlet I could find (saved games, Xbox temporary storage, reserved space). I deleted my rolling cache and changed it from 8gb to 16gb. I then re-downloaded/reinstalled the base sim, then quit MSFS and powered the console off. Turned it back on, and downloaded each WU/payware DLC in chronological order — but only two at a time, before repeating the quit/power off process. It’s been nearly two days now and everything seems normal — no lost upgrades and sceneries, etc.
Add your name to the thread linked above, so that MS prioritise the issue.