Background App Refresh not working

Hi, 

I have an iPhone XS running iOS 15.4.1.

I have recently noticed I’m not longer getting Notifications from key apps, including Whatsapp, and Arlo. There could be more.

I believe it’s related to Background App Refresh not working consistently. Only when I open these Apps do new messages/updates appear.

I intermittently get notifications if the App has very recently been opened, i.e within the last minute.

Here’s what I’ve tried so far, without success:

-Checked issue occurs on both Wi-Fi and Mobile data

-Updating IOS and Apps to latest versions

-Restarting iPhone

-Ensure Notifications and Background App refresh are switched on globally and specifically on for affected Apps.

-Turning off Notifications and Background app refresh, restarting phone and turning them on again 

-Turning off Focus mode, DND, Driving mode etc. I’ve even added these apps to the allowed lists

-Checking low power mode is off

-Backup and full restore of iPhone

It feels like this is a bug rather than a settings issue as I made no changes before the issue and an IOS bug as multiple apps are affected. 

I don’t want to delete the apps and reinstall them as I will lose key data too large to backup to iCloud.

Looking for suggestions of things to try. Is it worth trying resetting network settings or does this only wipe saved Wi-Fi networks?

Thanks