It looks like this is intended vBulletin behavior after all. I can't find any other reason for this happening except for the Spam/Akismet settings. Essentially, any new users that don't have moderator permissions and have post counts below a certain threshold have all new threads scanned against an external database. Depending on whether their post is determined questionable or not (could even be a result of their IP range), the thread will be placed into the moderation queue. The spam filters could be returning a false positive based on any number of factors: short posts with no punctuation, IP ranges, the browser's User Agent, the user's email address, or the username. It might be returning a false positive for users with email addresses matching the domain of the forum, thinking these were fake emails.
When this occurs, an entry is supposed to be made into the Spam log, but depending how often you check that log, it may have already been removed. It might also be removed if there is a bug with the code that cleans the log, but I haven't found that code in my perusal through the post data managers.
I think the bug mentioned in the OP was somewhat different, but was actually due to a bug fixed in RC 1, where the forum ID was not properly submitted when there was only 1 wiki forum the user could post in - this could lead to improper permissions being fetched. If they were all section edits, it also could have been a modified permissions array within the parser pass used to distinguish the sections. If this is the case, we've worked on making the parser more insular for portability's sake, and should have fixed several locations where this might have been possible.
I'm marking this as Fixed in RC 1.