BenQ XL2720Z White smudge/stain in the middle only on Displayport cable

Feb 8, 2019
I spotted a reasonably big (fingerprint sized, more circular though) 'smudge' in the center of my display that is NOT on the monitor physically (so it's not an actualy smudgey fingerprint thing) and I can't get rid of it. Only noticable on dark background like at night time in a game or just a flat black background in paint (On the screenshot) but I'm pretty sure it's there even on white background too.
The smudgey thingy only comes up when windows starts. When I reboot and I see the ASUS booting screen, the spot where the 'spot' is supposed to be isn't there yet. Right before the welcome/login screen comes up in windows the screen goes black for a split second: Also no smudge. Right after windows is live it is there though.
Here it is in a picture:

I use a BenQ XL2720Z monitor with a displayport cable at 1080p/144hz plugged into a Strix GTX 1080ti. I think the issue is with the DP cable. Changing the input to HDMI 2 the smudge goes away. Swapping back to DP and it's there again.
So far I tried:
Checking/cleaning the actual screen (obviously first thing was this, just in case)
Switching resolutions and refresh rates (no fix)
Messing around with monitor specific options like brightness and such (nope)
Using the DP cable the other way around (nada)
Rolling back to previous display driver (negative)
Testing the display with a HDMI 2 cable (which gets rid of the issue but I can't run at 144hz which is a no-go)

I guess the question would be this: What's borked?
-The DP cable
-The monitor
-The graphics card

I cannot get another DP cable before monday but that's gonna be the next step.
Any ideas other than try another cable?