Mercury109 left a comment (kamailio/kamailio#4169)
Thanks for the quick replies, I tested with master and looks good.
You're right, the BYE towards the callee is going out 7ms after the loopback BYE sometimes and instantly other times. The two calls compared were identical scenarios so I'm not sure why that happens. 
If we can add this to 5.7, that would be great. Thanks again!