“Restoring” my iPhone to factory via iTunes on a computer (as suggested by multiple Senior Advisors that handled my ticket), was the only thing that fixed the issue for me.
The reason why Apple is telling you to try different troubleshooting methods, is because the solution to this issue is different for every user.
Some users were able to resolve this issue by toggling off and on iMessage in Settings, this method didn’t work for everyone.
Others were able to resolve it by signing out of their Apple ID on the impacted device and signing back in. AGAIN, this method doesn’t work for everyone, and the list goes on and on for different methods that have “worked” for people.
Apple is very aware of this issue and has yet to figure out what is causing the issue and why it’s impacting some users and not all users but in the meantime while they’re investigating, some of us like myself have worked thoroughly with Apple Support running tests and etc to help pinpoint the issue and find which troubleshooting methods worked more permanently.
The only workaround that has worked for me, was restoring my iPhone back to factory using iTunes on a computer.
Resetting your iPhone via Settings on the phone itself, is only resetting the phone using the same corrupted software, while restoring via iTunes, puts new software on the phone.
Restoring the phone to factory via iTunes, is a requirement from the engineers, in order to advance your ticket to the next level with Apple. If restoring via iTunes doesn’t work and the issue comes back, then your Senior Advisor would need to work further with the engineers and you, to find the root cause and a more permanent fix.