With iOS 15.4 Beta 1 it now works again, but I am not sure if this is just because the iPhone rebooted.

I can reproduce this issue with my iPad and my iPhone both on iOS 15.3.

Is this also happening if you put your app in the background while your device is connected to the debugger (Xcode)? Is this also happening if you but the app in the background and immediately switch back to it, or only after some time? Maybe a background task completion block isn't called correctly?