I believe the expansions are platform based while seasons are character based. So If you had cross save active when you bought the package, with your PS4 account as the primary, then your season passes are tied to those characters. I’m guessing that’s why your PS4 has all the seasons active while your Steam characters do not.
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It wasn’t removed, it is assigned to your PS4 characters. If you reactivate cross save then the passes will be available on any system you cross save to, as long as your PS4 characters are your active ones. If you want to continue to use your current steam characters, then you would need to buy the pass again for that account.