Deployment fails when UEFI partition is on a separate disk to the root partition

I am running MAAS 2.4.2 under Ubuntu 18.04.

I want to deploy 18.04 using ZFS for the root disk onto a Dell PowerEdge R630 that has 2x500GB SAS drives in RAID 0 and a 500GB NVME SSD attached to a PCIe M.2 adapter. Unfortunately, the R630 cannot boot directly off of the NVME SSD and MAAS doesn't allow using zfsroot partitions in BIOS mode so if I want this to work I would have to have the UEFI boot partition installed on the SAS drives and zfsroot on the SSD. I have attempted this several times and commissioning works fine plus the installation succeeds but ultimately the deployment fails because I end up at the GRUB prompt and it doesn't boot into the OS post deployment so deployment times out and fails.

I have got this hardware configuration to work successfully with MAAS by using BIOS mode boot, creating a small /boot partition on the SAS drive and using ext4 for the root partition on the NVME SSD but really I want to use ZFS. There is nothing in GRUB or ZFS that depends on UEFI so I would like to see this restriction/requirement removed.

Is deploying with /boot/efi (in UEFI boot mode) on a separate disk to the root partition not supported? If that is the case, MAAS should prevent deployment from starting.