There's nothing else promoting people for sure. I'm sure it's XF system actually promoting people, but not as we would expect it to happen.
I'll try and summarise.
Auto-promotion is from Primary Group A (registered members) to Secondary Group B after 5 posts.
User is a member of registered members, posted at least 5 messages, registered for 7 days.
That is working fine for registered members.
When we imported the forum it maintained our original VB group names, so we moved all the accounts to Group A, and the correct secondary group for their status.
That's all confirmed in the account change log.
However, when users that are already members of both Group A and B log in for the first time on XF, it's detected as being an account upgrade somehow, by something. This account upgrade did not happen. There's nothing in the log, and in fact nothing to upgrade to or from. It just seems to be something that's triggered by them logging in.