Seeing all the replies in this thread it is good to know I am not alone. iPhone 7 same issues, particularly with WhatsApp. Not reiceiving notifications then one will slip through, I get all exited thinking it has fixed itself and there we go. Open the app to find 6 messages that I didn't know about. Actually have friends calling me to see if I am alright because they can see their message only has 1 grey tick.
Also keep getting told I have just signed in on a new device, when the device is not new at all. Just happened now when I came to log into this forum, using my Mac mini which I have had since 2010..... Thats a real head scratcher but it did remind me that it keeps happening with my iPhone too.
fingers crossed they get all this fixed. You are not doing your street cred any favours with this APPLE...
by the way thanks for the laugh C with the name changes for face, and twit.
Come on apple get this sorted.