OpenZFS provides ZPOOL features that can be not compatible with GRUB. To check compatibility zpool-features.7
ls -l /usr/share/zfs/compatibility.d/grub2 lrwxrwxrwx 1 root root 10 cze 29 15:54 /usr/share/zfs/compatibility.d/grub2 -> grub2-2.12 cat /usr/share/zfs/compatibility.d/grub2
zpool set compatibility=grub2 bpool