This is it. Firmware and OEM system configuration in the PC industry is basically garbage, and always has been. OEMs tweak until it boots whatever version of windows will ship on it, rush it out the door, and never touch it again unless some volume customer comes back with a bug report.

Linux needs to come in after the fact and run on whatever garbage happened to ship.

(FWIW: in this case the root cause was a host bridge in the tables which had been granted a truly outrageous memory space despite having no devices in it. It was likely a typo, or some test stuff that got left in.)