Add the setting: Wait X seconds for all drives to be displayed before importing the pool.

[6ax]

https://github.com/zfsonlinux/zfs/wiki/Ubuntu-18.04-Root-on-ZFS

At the bottom of the page see section: MPT2SAS

Most LSI cards are perfectly compatible with ZoL. If your card has this glitch, try setting ZFS_INITRD_PRE_MOUNTROOT_SLEEP=X in /etc/default/zfs. The system will wait X seconds for all drives to appear before importing the pool.

Same from archzfs:
mkinitcpio -H zfs

To change the seconds of time to wait for ZFS devices to show up at boot:

    zfs_wait=30

Tnx.

[dasJ]

But shouldn't that be fixed by the udev settle which happens before the pool import?