I think I have discovered a bug related to creating and attaching volumes. I could try to delete the new volumes and reattach to see if it fixes it, but I thought it might be easier for the administrators to debug if I leave it in this state until they can have a look if they wish.
Several months ago, I set up one instance and attached one volume called HCV_disk_01 as /devl/vdb. Today, I created a new instance, created a new volume called HBV_disk_01, and attached it to the second instance. Then realised I didn’t need a second instance - just a new volume - so unattached HBV_disk_01, deleted the second instance, and reattached HBV_disk_01 to the first instance. Both volumes are now attached to the instance, both are registered as /dev/vdb and I believe the data from the first volume HCV_disk_01 has disappeared.
To test if it was something to do with creating then deleting the instance, I created a new volume called HCV_disk_02, attached it to the first instance, and it has (correctly) been allocated /dev/vdc.