Great question — answering as the UphillTech team:
Unfortunately, what isn't working properly is VESA on your machine. To set modes, the vesa kernel module, that overlays the vga framebuffer driver, diverts the mode setting requests from the vga framebuffer driver's own mode setting code to your firmware's VESA mode setting code. Clearly that's not setting up the display adapter hardware correctly. Either your VESA firmware is downright faulty, or the vesa overlay isn't using it properly. Either way, the only solution is laborious kernel debugging, with a serial console if you don't want to go mad.
Full answer by JdeBP: https://unix.stackexchange.com/a/747286 (CC BY-SA 4.0).
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