Auto-Lock not working in iOS/iPadOS 15

Hi there

I'm using macOS Server (V5.11.1) with Profile Manager and Apple Configurator 2 (V2.15) to manage iPads and iPhones within our company.

All devices are running in Single-App-Mode (running a self-developed app). Further on all devices a auto-lock time of 2 minutes is configured.

Since we updated some of our devices to iPadOS 15 resp. iOS 15, the auto-lock stopped working. On older versions (like iPadOS 14.8) it is still working.

We already tried to solve the issue playing around with different settings - turned off "AssistiveTouch", "Guided Access", "Low power mode", "Focus", etc. but no effect.

Do you know if this is a known bug in iPadOS/iOS 15 or is it just a side effect of a new feature?

Thanks for your help! Rafael