Well, remember when I said:
Here you go, maildir syncing problems finally fixed (I hope). Well, except there's this "new-dir-only syncing" optimization which I'm not really sure if it works as it should.
Just before I found the expunge bug in index, I was trying to fix the new-dir-only syncing because the code looked wrong. Then I figured out the indexing problem and thought maybe this one wasn't broken after all, and then forgot about it.
Finally after lots of debugging, I came to same conclusion again. The code was wrong, and cause of the remaining "Maildir sync: UID < next_uid (312 < 3)" errors.
So, please try and see if you still can break this release. Other changes include some crashfixes for cache file and fix for mbox which kept constantly rewriting headers for \recent messages if we had selected the mailbox as read-only (EXAMINE, STATUS).