Humm.
Not sure since I don't force the registration queue and my test board is still on 7.5.6.
I would need to upgrade my test board and test it.

But I assume it could be the stop forum spam module in 7.5.7.
There are options on how stop forum spam reacts to registrations.
Quote
Spam Protection Level:
You have four choices; Manual ( initial setting ), Semi-Auto, Auto with message to Registrant and Auto with silent Fail. Each have their pros and cons.

Manual - This allows the Admin to easily check new registrations with one click
Semi-Auto - This automatically checks new registrations in member management, but might delay, if there are timeouts with StopForumSpam
Auto with Message - Adds to Semi-Auto and gives a new registrant a message that they were blocked and how to remedy the situation
Auto with Silent Fail - Does what Auto with Message does, but fails silently, which might fool spammers into thinking they signed up.

So possibly it could be deleting the actual registration as a disapprove???
Or you are getting some type of spam email being spoofed.
I would check the email server and see if the emails actually came from your server.

Also on the referrer check.
You might as well disable it on the next line.
The setting will disappear one day and it has been suggested in many posts to turn it off since it is no longer needed and today causes problems for some.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers