The issue is gone for me. I haven't received the annoying alert at any time in the past 36 hours. That's a changed condition, relative to when the problem first appeared, when it was intermittent throughout the day. There's no evidence of it today.
I'd like to think Apple fixed it, but I suspect that their IMAP system comprises such a massive number of servers that it would take time to ensure that the fix is applied to all of them.
The Bugzilla post (above) probably explains it. It wasn't a problem that affected all users universally. I have an associate in Arkansas who never had any problem accessing her message content. I guess it just depended on which specific server you happened to connect to at the time you tried to open and read your messages.