I do not have an answer,
Check the camera manual for info.
My D750 instructions might suggest that the shutter system is modified somehow as one press of the shutter is required to lock the mirror up, then second press fires the shutter.
Have you tried a remote cable release for the shutter, and does the camera fire with the camera shutter button?
The camera on it's own functions in Mirror Lockup normally. So it's not an issue of the camera not working as designed. It is only when tethered that LRCC will not fire the camera. If I take it out of Mirror Lockup - only then does LR fire the camera. I just wanted to confirm that this is a limitation of LRCC (or not) and what the solution is.
I am able to be tethered, in Mirror Lockup, manually fire the camera and LR will suck up and display the image. But for now it seems to be a no go to use LR in tethered mode to fire the camera from the computer.