I have run into this weird screen resolution issue, usually related toNvidia driver bug where the EDID fails to recognize the screen resolution. But I have an AMD card (7800XT) running with the Mesa driver "Mesa 26.0.5-arch2.4".
The problem comes up during boot, before the kernel is loaded when GRUB is loading. Even the BIOS splash is low resolution. I went down the GRUB and various configs rabbithole but ultimately it is fixed by disconnecting and reconnecting the displayport cable on the GPU and rebooting. Single monitor btw.
This is weird because I never had any similar issue ever using linux. I had Mint installed prior, it also uses GRUB and never had an issue. Maybe because of the graphical theme used in CachyOS? It definitely starts before the kernel is loaded and I know the graphical configs have no effect, only by reconnecting the cable the EDID is recognized.
Anyone have any thoughts on this?