The feature simply refused to turn on, forcing you to enter your PIN code when out in public constantly.  After updating to iOS 15, some users started to see prompts stating their iPhone’s storage was nearly full when they had plenty of space left. According to the release notes, that’s been fixed as well.  Here’s the full changelog from iOS 15.0.1: 

Unlock iPhone with Apple Watch may not work on iPhone 13 modelsSettings app may incorrectly display an alert that storage is fullAudio meditations could unexpectedly start a workout on Apple Watch for some Fitness+ subscribers

The iPad’s changelog only states that the update fixes bugs but doesn’t list anything specifically.  You can download the update by opening the Settings app and going to General > Software Update. If you find something that’s changed or fixed, feel free to leave a comment below.