This is one long discussion, took me some time to read through but with everything tried a couple of things stand out, a solution for one doesn’t seem to work for others and no one seems to have tried an iTunes Restore as New or if they have they haven’t reported back!
In my opinion if the issue was just iOS related then every specific iDevice would have the problem, ie if one iPhone X is effected then all will be, but my X has been fine. That would tend to indicate the problem has come about from either the Update process conflicting with what’s already on the users device (data, caches etc) or how the Update has actually been applied (OTA or iTunes).
To rule out both of the above can someone do the Restore as New to see what happens, I appreciate it’s a big ask but with iCloud most of your data and photo’s will come back once signed in again with your Apple ID (if things have been setup that way in advance). If a current iTunes BackUp is also archived everything can be returned to “normal” in the event the Restore doesn’t resolve.
The above is just my view on this situation but on page 5 someone did report being given an exchange unit by Apple and even though that was still on 11.4 all was ok, I’m assuming because the new device was devoid of any junk, and even when adding the Apple ID I guess every continued to operate as expected.
As I said, just my view, but might be worth a try.
