There should be separate Orientation settings for the main and cover screens. Personally I would always want the main screen as Auto rotate but the cover screen locked to portrait. It's annoying that I have to keep switching this setting whenever I change from one screen to another. Not sure how others feel about this. We could have it in the same place we set Orientation for the lock screen.
As soon as I received the Fold 3 on launch day I installed Tasker and added a Profile to do this for me. Have a Google and you should be able to find out how to do it but if not then let me know and I will send you my Profiles.
Tasker is a great tool and you should have it anyway too be honest.
I know this post was while ago, so you may have got sorted already, but there's a Bixby Routine for this. It's called Cover Screen Portait View. It simply turns auto rotate off when the Fold is closed, and back on when you open it up.
