You might want to post over on vbulletin.com in the server forum
https://www.vbulletin.com/forum/foru...-Configuration to make sure it's not some configuration issue before changing the code.
ETA: Just thought I'd add, it's OK to ask your question here as well, someone here might have some idea. I was just thinking that if you have everything set up correctly you might have found a (possibly known) bug or issue.
The obvious thing (which I'm sure you thought of) would be to take "_slave" off the call you highlighted in red, but then obviously you start to undo the benefit of having a slave server.