revq did you find any solution? Having a similar problem right now.
I've set up automatic decryption with an usb drive. After adding the additional key to both of my drives, one of them just disappeared. I am able to manually unlock and mount it but in omv it's only shown as device and the volume is missing. When manually decrypting and mounting it, it is no longer marked as missing and shows the storage usage.
But I don't think that it is solely a omv issue. "blkid" only shows one of the drives (both after manual unlock) too.