Dear maintainer, It seems that the same symptoms are currently occurring in my system. I'm running buster with OpenZFS 2.0.3 from buster-backports, and have upgraded my root pool with the latest features. My /boot is not on ZFS. Update to grub-efi-amd64 2.02+dfsg1-20+deb10u4 rendered my system unbootable and indeed the issue seems to be missing pool name in the generated kernel command line.
I can work around the issue by hard-coding the correct pool name instead of using this grub-probe line in /etc/grub.d/10_linux : rpool=`${grub_probe} --device ${GRUB_DEVICE} --target=fs_label 2>/dev/null || true` -- Br, Markus Vuorio