The system I had encountered the problem on had been freshly upgraded from
stretch. It turned out that while the GRUB *packages* had been upgraded, the
boot loader in the EFI system partition was still from stretch. I had to
explicitly
grub-install --target=x86_64-efiAfter this, things are booting fine. So: GRUB issue, fixed in buster (but do pay attention to the version string on the GRUB menu screen at boot!)
