I knew the acronyms already, but UEFI is a pretty huge standard, and CSM does provide quite a large set of “compatibility services”. So that does not help in itself to pinpoint what the problem would be and how we can address it.
Not sure at what it is better in “Legacy BIOS” mode (which is supposed to go away from new hardware as we speak). We boot into xen.efi or grubx64.efi, from a GPT disk. And the QubesOS install USB image, which boots to black screen on my laptop, perfectly boots on another full-AMD full-UEFI platform. That should explain that I don’t buy this argument 
OTOH, a Debian Live USB stick does boot on the laptop in UEFI, so you may still have a point