It sounds like your Xbox account was linked to a different EA Account and you relinked it to the same EA Account that held your Steam data. A situation a bit like this:
Before Relinking
EA Account #1
EA Account #2
After Relinking
EA Account #1
EA Account #2
- No platform accounts linked
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Keep in mind that all progress is stored on the EA Account and not the Platform account. Meaning that when you relinked your Xbox from Account #2 to Account #1, it didn't bring over any progress, as it would still be stored on Account #2. This means that your Xbox account would be able to access the data from Account #2, hence why you would see the progress you made on Steam, but not Xbox.
Currently there is no way to move data between EA Accounts, so if your Xbox and Steam platform accounts were linked to different EA Accounts at the time, it would not have brought over any data.
This was a bit more wonky before Cross Progression rolled out, so if you unlinked a platform account before Cross Progression was rolled out on your account, you had a decently high chance of losing your in-game content. Multiple warnings about this were posted. So it's still possible that you can get your Xbox progress back if you were to link it back to the EA Account it was originally linked to, though it's also possible this progress has been lost because of that.