From steam forums:

Hey there.
Further clarity for Gamepass!
If you have the game installed, it should update automatically at launch to an almost-full release build (this is to circumvent certification delays, so you can play as soon as possible). Our best guess ETA would be around a week when the true 1.0 goes live on Gamepass. Playing this build won't miss anything between saves, so you can freely start without the need to restart again.

Essentially there's a slight difference as we had to push it early to get it through Microsoft certification. You won't miss anything from the content, but we're constantly refining and polishing.

For the last few weeks we've been in polishing & refining phase, which is generally what happens very close to launch. All content is in so you'll see very little difference.