Bought a game while I had a Ultimate subscription

Ok thanks, I'm still new to all this subscription thing 😅

Yes, you can, as you now own it.

It sounds like you bought a Game Pass Core card and added it while you had an active Game Pass Ultimate subscription. In order to have gotten the full 12 months, you would gave needed to either cancel your Ultimate subscription or wait for it to expire before adding the Core card. Since it's already been redeemed, there's no way to change it back.

Did you have a regular Gold and/or Game Pass subscription when you bought Ultimate? Any remaining time you had would have been upgraded to Ultimate with the purchase.

Sounds like you added a Core subscription while you had an active Game Pass Ultimate subscription. In that case, Core will convert to Ultimate at a ratio based on price rather than time. 12 months of Core equals 4 months of Ultimate. If you completed the transaction, it cannot be reverted.