Bug #445367 seems like a duplicate of this. As described there, /etc/grub.d/30_os-prober ignores boot partition reported by os-prober.
/usr/lib/linux-boot-probes/mounted/40grub and /usr/lib/linux-boot-probes/mounted/40grub2 are the files prepending the "/boot" to linux kernel/initrd lines.
Bug #445367 seems like a duplicate of this. As described there, /etc/grub. d/30_os- prober ignores boot partition reported by os-prober.
/usr/lib/ linux-boot- probes/ mounted/ 40grub and /usr/lib/ linux-boot- probes/ mounted/ 40grub2 are the files prepending the "/boot" to linux kernel/initrd lines.