XPS 13 Plus 9320 with WD19S, dual monitors doesn't work correctly?

I have a setup with a BenQ RD28UA as my primary monitor, and a Samsung PB27U in portrait as my secondary monitor. Connected to two computers using a KVM switch. I use only DisplayPort cables, both cables and KVM switch are rated for 8k@60.

This works very well when using my Linux workstation, but not when using my XPS running Windows 11.

On the XPS and Windows I get full resolution on my primary monitor, but only 600x800 on my secondary monitor. The drop-down to change the resolution in the display settings is disabled. It doesn't matter what I try to do (and I tried everything I could find in web-searches). Firmware and BIOS is updated on both the dock and the laptop, before the system would not even attempt to connect to my primary monitor. And as I understand it should work with dual 4k@60 monitors connected.

The lid is forever closed on the laptop, since it's broken (something with the motherboard is damaged beyond repair, so the screen only works with the lid barely open, and the camera doesn't work at all). Everything else seems to work just fine though. Could this be an issue?

Or could it be that the primary monitor have a resolution of 3840x2560 pixels, i.e. just a little more than traditional 4k resolution, and that somehow pushes the bandwidth just beyond the limit? But even if I lower the resolution of the primary monitor it doesn't help.

I'm at my wits end, so please I hope someone could help me.