Was this a common thing people were reporting? I haven’t seen any other mentions of it until your post just now, but I was getting a ton of that over the past week on my iPhone and my iPad. Really freaked me out as I have dealt with account intrusions in the past. However I could not see any unidentified devices under my account in settings when I went to check - and I checked multiple times.
heard it now a few times .. they must have changed some server setups especially in regard to contact key verification and iMessages