I had a similar situation and will provide this information incase it helps others. You can see what my hardware is in my build info.
I set-up my system with a single monitor on 10.14.5 with a fresh install. Everything running fine, no complaints. Single monitor running on HDMI cable. I changed that single monitor over to using a DP cable and added a second monitor with another DP cable. I had only one monitor turned on and started the computer. Nothing but black screen........until login password screen appeared. Puzzling, but I continue testing things and found out that when I turned on both monitors, the BIOS splash screen would appear on one screen, followed by the Clover boot screen, then the Apple logo with progression bar, but when the Apple logo screen with progression bar got to about 50%, and a quick screen flash, that monitor would then go black and the other monitor now appeared with the Apple logo and finished the progression bar arriving at the login password screen. I would not ever have known this without turning on both monitors.
Again, after a series of tests, I found that one of the two DP ports on my graphics card will initialize before the other to the monitor. So, it is like one of them is designed to be the default port/cable to see a monitor first. But, once the OS and all the behind the scenes things happen and get loaded, the system preferences takes over and will finish with the monitor I have designated as the primary monitor. Once I realized that, I then had to go to system preferences and set the correct and corresponding monitor to that cable to get a proper result. Final result is, turn on computer, regardless if one or both monitors are turned on, the main monitor I use will show the entire boot process, Bios splash screen, Clover boot screen, Apple logo with progression bar and finally password screen.
I hope this is not just my issue but helps others along the way
Thaigyver