This is a fresh problem for me and I am trying to understand why the MX reverts to IP 126.96.36.199 when this issue occurs.
My setup includes a MS switch between the uplinks and the ISP.
The switchport connected to the primary MX uplink, will see a mac address cc:03:d9:9b:XX:XX while the MX boots up, which corresponds to the mac the MX pair uses in it's inside LAN and this confuses me even further...
This seems to be go away while deploying the devices in their final setup, so I'm starting to think that there seems to be something odd happening with the setup I'm using for preconfiguring the devices. I'll have to further look into it.