Since iOS 16 Beta 1 and in iOS 16 Beta 3 as well the reloadAllTimelines() method of WidgetCenter no longer reloads the timelines if the app is backgrounded.
I’ve also filed FB10432865 a couple of weeks ago.
reloadAllTimelines() doesn’t reload the widget if app is backgrounded
The issue is still present in iOS 16 Beta 4.
In previous betas, this also affected the iOS Music app and prevented it from updating the widgets live as the now playing song changed, but that seems fixed for the iOS Music app in beta 4, yet it's not working for third-party apps.
Another FB for this: FB11522170