iOS15.1 not available on iPhone 13 Pro

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

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