Solved.
The problem is only reproduced when booting the machine with the installation media plugged into the USB. The solution is to boot the machine without the installation media connected; wait for the flashrom GRUB to load; plug the installation media into the USB; and then boot from it.
I’d normally assume there is an isolated issue with my hardware/BIOS, but as I mentioned, Debian live doesn’t have a problem booting with the same setup.