Hey ExpandapantsI,
Just to clarify what seems to have happened here, progress is stored on the EA account level rather than on individual platforms like PlayStation, Xbox, Steam, etc. This means if you link your platform to a different EA account, you'll lose access to that progress.
In this case, it sounds like your PlayStation account was linked to a different EA account previously. That would be why you're not seeing that progress on the EA account you've just linked with your Steam and PSN accounts.
You would need to try re-linking your PSN account to its original EA account to see if that content can be recovered, and then you'd need to link your Steam account to that same EA account rather than a new one.
If you run into any trouble with that re-linking process, please contact support directly so they can take a look and help make sure the correct accounts are linked going forward. Thanks!