U2520D, left side is darker

I got a U2520D monitor a few days ago. I mainly use it for word processing, i.e. two white pages next to each other for much of the time.

Soon after starting to use it, I noticed that the left half of the screen is a little darker. Just enough to be noticeable and therefore distracting during work. In the evening, the difference is noticeable during any task.

This does not seem to be normal. I certainly expect more from a €400 monitor, especially from a U series.

I did not run any diagnostics yet because I don't know if the factory reset would erase the factory color calibration.

It is connected to a Surface Laptop 3 with Intel Iris via USB Type-C. All drivers are the latest.

As the difference is slight, it does not show particularly well on the photo that I managed to take (see below).

Please advise.