RenderOffscreen large resolutions not working

Hi everyone,

I have come across an odd issue.
In my level blueprint I “Set Screen Resolution”, “Apply Resolution Settings” and “Apply settings” to set the resolution to portrait format 8k (4320x7680) or 16k (8640x15360).
I start my packaged game from command line (Windows 11, UE 5.0.3) and pass -RenderOffScreen.
The game automatically saves out some renders (similar code to the PixelStreamingPlugin and HighResolutionShot) and quits afterwards.
All works perfectly on one machine, i.e. I get perfectly sized 16k jpg saved out.
But on 2 other machines I just cannot get it to give me the accurate resolution.
It seems to clip it to the resolution of the extended Desktop (i.e. two landscape format 4k screens) and I end up with a small portrait render in the center, with big black bars to the left and right.
All machines have two 4k monitors, almost identical hardware, all Windows 11, etc.
I just cannot figure out what’s causing the issue on the other machines.
Is there some Windows / Bios setting or additional flag I should use?

This is really obscure.

Thanks so much in advance.

Cheers,
Sebastian