I have received the monitor and tested it. It appears that the panel was indeed exchanged, but while the backlight uniformity is not that extreme anymore, the issue with the darkness in the upper left corner is still there, also meaning that the contrast is worsened there.
Because of this darkness issue, I am not sure yet whether I will keep the monitor or ask for a refund, it is just that it hurts the image quality and contrast in the upper left area quite a bit for me.
And also: The issue with the darkness in the top left area is not only present at 180Hz. It is also present at 144Hz and 60Hz.
Here are pictures of the new panel:
The grids are not visible at the top:
Here is a white background, notice how in the top left area, it is not white, but has more of a brownish color.
Here is a black background, notice that at 180Hz, the much stronger backlight bleed in the bottom right area (that we already know about) can be seen pretty well.
Edited by Neon Lights - 8/3/16 at 12:36pm