Buggy GRUB box border drawing on EFI system with Secure Boot enforced: �

Since a while, at least on 1 of my test laptops that uses EFI boot, the frame surrounding the GRUB menu is drawn using � chars, instead of lines/frames-drawing chars. Wrong encoding configured? Missing or incorrect font configured? Missing or incorrect font on the ESP?

In our GRUB menu, I requested a command line, and there:

grub> loadfont $grub_dir/unicode.pf2
error: prohibited by secure boot policy.

But loadfont unicode works and fixes the menu. That's basically what my Debian sid system does.