How may I connect my Apple Watch Series 3 with my iPhone 6s plus?

I encountered issues pairing my Apple Watch Sport(watchOS 4.3.2) and Series 3(watchOS 8.6) with an iPhone 6S Plus running iOS 15.8.4.

The Sport paired successfully on iOS 13.x, but after updating to 15.8.4, neither watch can complete pairing.

Behavior: the app scans the watch face and sets the wrist preference, but crashes during the connection/activation step. Upon reopening, it shows “unpairing Apple Watch.”