Storage Spaces Direct -- virtual disks shown as "no redundancy" and "unhealthy"

for no apparent reason I have a s2d virtual disk which has an operationalstatus of "no redundancy" and healthstatus "unhealthy". Running repair-virtualdisk gives this response:

PS C:\Users\administrator.PAO2K> get-virtualdisk "volume3" | repair-virtualdisk
repair-virtualdisk : There is not enough redundancy remaining to repair the virtual disk.
Activity ID: {a366982d-b602-4deb-8f6d-c8ec59c12217}
At line:1 char:29+ get-virtualdisk "volume3" | repair-virtualdisk+                             ~~~~~~~~~~~~~~~~~~+ CategoryInfo          : NotSpecified: (StorageWMI:ROOT/Microsoft/...SFT_VirtualDisk) [Repair-VirtualDisk], CimEx
   ception+ FullyQualifiedErrorId : StorageWMI 50001,Repair-VirtualDisk

PS C:\Users\administrator.PAO2K>

so how to fix? this virtual disk is tiered with triple mirror performance tier and double parity capacity tier, and is an ReFS volume.