No display output on mini Displayport (mDP) until login screen and startup manager is never visible

I have a mini-displayport (mDP) connected to my Mac Pro (2013 model, custom, Quad Core Xeon, Dual D500 GPUs, 16GB RAM, 500GB SSD).

There are no other displays connected.

This display is a 4k monitor, 27MU67, IPS by LG, and comes with documentation proving it is OSX Compatible.

1. The Apple logo and loading screens after the 'chime' are never displayed. Ever. The monitor remains at 'no signal' until the OSX Login Screen.
2. By holding 'ALT' at startup, the startup manager never appears - the monitor remains at 'no signal'. I know that the startup manager is really there because if I press right arrow key twice and then enter (I have Windows 10 on bootcamp) - eventually the Windows 10 login screen appears.
3. No Windows loading screens appear before the login screen.

I have tried 'power flushing', CMD+ALT+P+R on startup, plugging the monitor into all of the 6 thunderbolt ports. The monitor behaves normally as soon as the computer is a one of the login screens. I do not believe the monitor to be faulty, it's new.

It's also worth noting that if I connect the monitor via HDMI, the screens appear as normal, HOWEVER due to oversight on Apple's part, installing a low spec HDMI port, the monitor will only run at 30Hz over HDMI vs 60Hz over mDP. If I disable mDP 1.2 at the monitor, the monitor only runs at 30Hz for mDP as well, however the boot screens still do not appear.

It looks then, that the Mac will only output via HDMI while starting up. This is massively inconvenient as I don't want to be swapping over cables every time I start up and want to enter startup manager, recovery partition, hardware diagnostics, Windows, Backup Harddrives, etc, etc