3rd Party Notifications Bug

The problem I’m having since updating to iOS 15 has been that most of my third party apps/mainly messaging apps such as WhatsApp, Messenger, Signal, YouTube notifications haven’t been alerting me when my iPhone is locked. It seems to “hang” on not being delivered. It’s only when I open these apps that I get all my messages come through. I’ve factory reset and restored my phone and turned it off and back on again, but this problem still persists with it only working sporadically. I can attach a screenshot from a family member that sent me a Messenger message and it just stays on being undeliverable. Then it gets delivered when I open the app. I’ve checked all my notification settings are switched on also and that focus modes are off (which I never use anyway). This happens with all of my app notifications except from iMessage/text message and phone calls. Does anybody else have this same issue? Any help would be great to get this fixed?