It looks like several people immediately after receiving their iPhone 13 series reported that the phone did not unlock even if you had an Apple Watch on your wrist. Now it looks like Apple has started sending out iOS 15.0.1 to iPhones.
On the one hand, this problem is fixed, but Apple also mentions that “they fixed one or two bugs” that the first version of iOS 15 contained. The update must be available to everyone iPhones who can download iOS 15!
