No guaranty but try issuing this command mdadm --assemble --scan from a terminal window as user Root. You may have to mount the file system once you issue the command.
I just wanted to update this thread since I was having this same problem running two 1TB USB drives in a Mirrored RAID 1 on a Raspberry PI 3. I have been able to do a work around by setting up within OMV 3.0.88 a Scheduled Job to run at Reboot to execute the command "mdadm --assemble --scan" as user Root. This did work for me for the Mirrored RAID 1 to show up under the RAID Management and File Systems upon a reboot. It also remounts as part of the reboot process as well.