I stumbled across this issue yet again in a fresh buster installation. Various conversions did not solve it for me, but after reading [1], I checked and found that my initramfs indeed did not contain raid1.ko.
Adding raid1 to /etc/initramfs/modules made my system boot. This suggests /usr/share/initramfs-tools/hooks/lvm2 might be at fault here. I have to check whether this applies to bullseye/sid as well, but I guess it does. Cheers, sur5r [1] https://unix.stackexchange.com/questions/187236/grub2-lvm2-raid1-boot
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