Due to a powercut my system rebooted.
When it did, I had no Docker containers, and when I checked, my OMV share drive was missing.
I have rebooted back to use 5.4.106-1-pve #1 SMP PVE 5.4.106-1 and it works. I used that as a panic default, but I can see in logs I was under 5.4.114-1 before.
However, under 5.4.119-1-pve, when I start, it fails to mount OMV data partition.
mdadm --assemble --scan then reports /dev/md0 is already in use
Clues? (also to warn others!)