I have solved the problem of not receiving message notifications/alerts while my iPhone is locked and asleep, for myself at least.
Apologies in advance for the length of this post.
This issue has been affecting me for well over 6 months now and I just could not work it out.
I had tried all the suggestions in this thread, trawled through the interweb for hours trying various fixes, resetting all settings, complete wipes and restores from MacBook backups, fresh installs of iOS while in Recovery Mode etc, all to no avail.
I even spent a few hours talking with Apple Support staff on a few occasions.
It was during one of those calls that the problem suddenly seemed to right itself without explanation, as we hadn’t done anything drastic.
Coincidentally, my wife had just gone out to pick our son up from his Auntie’s, from whom I then received a text message with audible alert and banner appearing on a locked iPhone screen. Amazing! It’s working again, I thought.
Alas, the problem came back later that evening and continued during the following days.
This got me to thinking and realising that this only happens when my wife is in close proximity.
Last year she bought an Watch. Last year I started leaving my Bluetooth on, due to running the UK COVID-19 exposure app which relies on Bluetooth handshakes between devices that come within range.
We were sharing the same Apple ID as this suited our needs with Photo Stream, Notes and Calendar automatically going to each other’s devices without the need to manually share.
I realise that this is not the way that Apple want us to have our devices configured.
I did several tests with Bluetooth on and off. Bluetooth off on my iPhone resulted in message alerts functioning as should.
I then did message tests with my phone signed out of Apple ID and wife’s phone signed in and then vice versa. Results were that message alerts and notifications functioned correctly while my phone was locked and asleep.
The watch has never been linked to my phone and my belief was that it was linked to a specific device, but now I am wondering if it is per Apple ID.
Could it be possible that alerts were trying to be sent to my wife’s Watch somehow but just couldn’t make it?
We all know by now how the notifications appear only on Watch and not on iPhone if you are wearing it. Maybe it has to have a passcode and be unlocked. I am no watch expert as I do not own one.
So, to conclude, my wife’s iPhone and mine have since got divorced. She has her own Apple ID and running her watch alongside and I am now receiving message alerts/notifications normally and as expected while my phone is locked.
I did have to go into Settings on the watch and then the Apple ID seemed to sync to her new one.
My iPhone: 11 Pro Max running iOS 14.6
Wife’s iPhone: XS Max running iOS 14.6
Wife’s watch: Series 5 running watchOS 7.5
I hope this post gives you all some ideas or maybe solves the issue for some of you. Good luck.