MacBook Pro 15" 2018 Display not working.

This actually sounds less like the display is not working and more like the computer is not booting all the way, like it's failing to POST (Power On Self Test). When you said you've done a bunch of diagnostic tests, just for point of clarification, you've done stuff like an SMC reset or NVRAM reset?

This could be lots of things, and without knowing some extra stuff for sure, it could be difficult to say, but I would wager logic board problem. To narrow down though, I would open up the machine, disconnect everything aside from 1 of the USB C boards (so you can plug in to power on). See if it does the same thing. I would also check with this with the battery connected since sometimes these 15" models can be fussy about turning on without battery plugged in.

If it's still doing it with nothing else connected, chances are it's the board. I have a couple of suspicions, but don't want to conjecture without more info. If you venture to getting the machine open, post some pictures of the internals in your original question!