[RPi4] No video output on HDMI-0

OK. I also guess it's the splitter.

Quote from Luvide

is Windows getting that information another way?

There is no other way of determining the display capabilities.

Are you sure windows can see the full list of display modes?

i.e. if you change display resolution in windows, do you get the same list of resolution/refresh rates both with and without the splitter?