Hi all!
I've got the same issue, fresh OMV 5 install under Proxmox VM. I have a bit more complicated hardware: two pass-through 8TB HDDs and two pass-through 8TB USB HDDs (for backup). After a lot of research, this is what solved the issue for me:
Code
# echo 'RESUME=UUID=<the UUID of your boot drive here>' > /etc/initramfs-tools/conf.d/resume
# mdadm --detail --scan >> /etc/mdadm.conf
# omv-salt deploy run mdadm
# update-initramfs -u
# update-grub
Then reboot OMV. That's it! Hope it could help someone else.