BTW: I can cornfirm that adding `GRUB_DISABLE_OS_PROBER=false` to `/etc/default/grub` does *not* solve this problem.
- Bug#960565: grub-common: update-grub ignores os-prober whe... Leandro Doctors
- Bug#960565: Acknowledgement (grub-common: update-grub... Leandro Doctors