OMV5 + ZFS: Unable to add mirrored vdev to pool

Fresh install of OMV5 with proxmox kernel (1) and openmediavault-zfs (2) from omv-extras.org. Trying to create a ZFS pool with four mirrored vdevs (2x12 TB + 2x12 TB + 2x8 TB + 2x8 TB).

I'm able to create the pool with the first mirror vdev, but expanding the pool with second set of mirror drives fails. Error message: A mirror must contain at least 2 disks. Yes, I am selecting two drives when expanding the pool

Confusingly, zpool status seems to be OK?

root@omv-nas:~# zpool status
pool: zfs-tank
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
zfs-tank ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV310R4 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV3B4T7 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV24C09 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV2LDGN ONLINE 0 0 0

After the attempt to expand with a second set of drives, the pool is missing from the ZFS overview. Also, the message 'A mirror must contain at least 2 disks' immediately pops up when selecting ZFS.

Should I expand the pool using CLI instead of GUI, or what am I doing wrong here?

Thanks for any pointers in the right direction!

(1) Proxmox kernel installed because the 5.2 kernel in backports is currently uninstallable: linux-image-amd64 depends on linux-image-5.2.0-0.bpo.3-amd64 which is missing from the backports repository.

(2) openmediavault-zfs installation initially failed because zfs module wasn't ready when setting up zfsutils-linux. Fixed with modprobe zfs; apt install.

Thank you for responding!

After rebooting, the pool is still missing from the ZFS overview. Also, the message 'A mirror must contain at least 2 disks' immediately pops up when selecting ZFS. zpool status still seems OK.

Is the ZFS GUI unable to handle a pool with several mirror vdevs? Maybe I need to use a couple of RAIDZ2 instead (4x12 TB + 4x8TB).

Edit: Creating the pool by CLI and rebooting: Same result. No pool in ZFS GUI, error message pops up, zpool status looks OK.

@subzero79 Thanks a lot -- nice to know that the issue will be looked into!

For now, I decided to create the pool using CLI and ignore the ZFS GUI until the bug is fixed. Filesystem shows up fine in OMV -> Filesystems, after all.