I have the same issue with booting straight into memtest. 

I wish I knew why "set -u" was added to /etc/grub.d/10_linux_zfs as it's easy to see that it was added; it is not easy to see why it was added.

"set -u" appears to be a significant factor in why zfs grub is failing.

sudo grub-mkconfig

[...skip some output...]

### BEGIN /etc/grub.d/10_linux_zfs ###
/etc/grub.d/10_linux_zfs: 229: version_sort_sort_has_v: parameter not set
Warning: didn't find any valid initrd or kernel.
### END /etc/grub.d/10_linux_zfs ###


grub-mkconfig works if I comment out the 'set -u' like this:

$ diff -u  ~/10_linux_zfs-grub-4ubuntu1 10_linux_zfs
--- /home/user/10_linux_zfs-grub-4ubuntu1	2023-08-29 08:53:42.056383448 -0400
+++ 10_linux_zfs	2023-08-29 08:53:57.896516515 -0400
@@ -30,7 +30,7 @@
 export TEXTDOMAIN=grub
 export TEXTDOMAINDIR="${datarootdir}/locale"
 
-set -u
+# set -u
 
 ## Skip early if zfs utils isn't installed (instead of failing on first zpool list)
 if ! `which zfs >/dev/null 2>&1`; then



$ sudo grub-mkconfig > /dev/null
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
jFound linux image: vmlinuz-6.3.0-7-generic in rpool/ROOT/ubuntu_via8pv
Found initrd image: initrd.img-6.3.0-7-generic in rpool/ROOT/ubuntu_via8pv
Found linux image: vmlinuz-6.2.0-21-generic in rpool/ROOT/ubuntu_via8pv
Found initrd image: initrd.img-6.2.0-21-generic in rpool/ROOT/ubuntu_via8pv
Found memtest86+x64 image: /BOOT/ubuntu_via8pv@/memtest86+x64.bin
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
Adding boot menu entry for UEFI Firmware Settings ...
done


$ sudo dpkg-reconfigure grub-pc
grub-pc: Running grub-install ...
Installing for i386-pc platform.
Installation finished. No error reported.
  grub-install success for /dev/vda
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: vmlinuz-6.3.0-7-generic in rpool/ROOT/ubuntu_via8pv
Found initrd image: initrd.img-6.3.0-7-generic in rpool/ROOT/ubuntu_via8pv
Found linux image: vmlinuz-6.2.0-21-generic in rpool/ROOT/ubuntu_via8pv
Found initrd image: initrd.img-6.2.0-21-generic in rpool/ROOT/ubuntu_via8pv
Found memtest86+x64 image: /BOOT/ubuntu_via8pv@/memtest86+x64.bin
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
Adding boot menu entry for UEFI Firmware Settings ...
done

Once 'set -u' is commented out, the system boots from zfs and displays the login screen instead of memtest.

Versions I tested with:

$ dpkg -l | grep grub
ii  grub-common              2.12~rc1-4ubuntu2        amd64  GRand Unified Bootloader (common files)
ii  grub-efi-amd64-bin       2.12~rc1-4ubuntu1        amd64  GRand Unified Bootloader, version 2 (EFI-AMD64 modules)
ii  grub-efi-amd64-signed    1.194+2.12~rc1-4ubuntu1  amd64  GRand Unified Bootloader, version 2 (EFI-AMD64 version, signed)
ii  grub-gfxpayload-lists    0.7                      amd64  GRUB gfxpayload blacklist
ii  grub-pc                  2.12~rc1-4ubuntu2        amd64  GRand Unified Bootloader, version 2 (PC/BIOS version)
ii  grub-pc-bin              2.12~rc1-4ubuntu2        amd64  GRand Unified Bootloader, version 2 (PC/BIOS modules)
ii  grub2-common             2.12~rc1-4ubuntu2        amd64  GRand Unified Bootloader (common files for version 2)