Did you already file a bug report via Feedback app to Apple?

I have the same issue. There seem to be 3 constellations:   users are not affected by the problem at all (best case). the delivery of events in the background is unreliable compared to iOS 14. no events are delivered in the background (worst case).   It looks like this is a bug in the context of the new Focus Mode of iOS 15 and/or the App Switcher.

I have the same issue and it is not yet fixed in iOS 15.1.