iOS15.1 not available on iPhone 13 Pro

OK. Not sure where you got it. This is what Apple says about iOS 15: https://support.apple.com/en-us/HT212788

The current iOS version is 15.0.2. iOS 15.1 is a beta version.

The fact that you installed it doesn't change the fact it's a beta version of iOS. It just means that phone is enrolled in the beta program.

Apple Beta Software Program