I'm hitting the same bug on Stretch with ext4 on lvm2 on mdraid1x on mbr partitions. The part_msdos, diskfilter, and mdraid1x modules are missing from grub.cfg generated by update-grub. Booting fails with a grub rescue prompt, requiring rescue media to repair grub.
I can insert those modules manually using GRUB_PRELOAD_MODULES="part_msdos diskfilter mdraid1x" in /etc/default/grub, but they should be automatically detected and added to grub.cfg. Daniel Bakken
