I have 2 identical phones (Xr) on 2 different apple IDs and both paired with an apple watch 3. Both were updated from IOS 13 to 14 at the same time; same for the all next IOS 14 updates.
One phone shows no problem at all, the second one is suffering with the notification problem. Some apps work OK now (WhatsApp), mail notification come in grouped 4 to 6 at a time, usually 15 mins or more after the last email was received.
No changes observed with the IOS 14 updates, I remember WhatApp was problematic initially, not sure if a WhatApp or IOS update is responsable for WhatApp working OK now.
Tried all proposed solutions here and other fora, except for turing off bluetooth and reset then setup iphone as new.
None of solutions solves the issue. Un-paring and erase the Apple watch, the pairing again, which took a lot of time solved nothing.
There doesn't seem to be much of a respons of Apple, on this which may have it ending up in the same bin with the OSX audio stutter with bluetooth problem: not getting resolved. Looking forward to Apple putting cars on our streets...