Weird compatibility issues????

[MaorRX]

I had a recent problem booting up a my new coffelake PC i build . I was using an AOC Freesync 75 hz monitor and a AMD RX 580 8GB edition. When i used Display port on the monitor, the computer would not pass the bios screen, it would just not want to boot, or post, it would freeze in bios screen. When i used the exact same monitor, but with HDMI, it posted and boot fine. I thought maby there was a problem with the GPU with the display port slot, I tried a different GPU (A NVIDIA GTX 950) with displayport on the same monitor and it boot. Then i was sure there was something wrong with my RX 580, but then I tried a different monitor (A Asus VP279) with display port with the RX 580 and it boot.Asyou can see both the monitor and GPU were fine, they just have a compatibility issue.

Why is that??

Another thing i noticed was that the debug led on the computer was still on when the AOC monitor was plugged in to the computer, even when  I unplugged it from the wall, but as soon as i unplugged the displayport the debug led on motherboard when instantly off. It was like the display port cable was somehow drawing power into the motherboard( I dont see this issue on when the new monitor Asus monitor is plugged in).

Really wierd issues.