On a working OCP/MTV setup which previously successfully migrated several VMs from eco-vcenter's eco-iscsi-ds3 datastore, I opted to extend the storagemap to include a recently added eco-iscsi-ds4-vmfs vmfs5 based iscsi datastore.
While editing the storage map, adding a mapping looked OK I was able to select the new "eco-iscsi-ds4-vmfs5" store and populate all of it's correct target/secret/vendor. Once I clicked on "update mappings" I noticed the plan had and issue:
SourceStorageNotValid
Source storage not found.
To troubeshoot, check the Forklift controller pod logs"

OCP 4.18
MTV 2.9.0

First time I hit this

1. Have or create a storage map with one datastore, my initial map only referenced eco-iscsi-ds3
2. On vmware create a new datastore, in my case added eco-iscsi-ds4-vmfs5, I don't think/suspect the fact that it's vmfs5 rather than vmfs6 should cause this but I might be wrong. 
3. Edit the existing storage map, attempt to add a new mapping
Source: eco-iscsi-ds4-vmfs5
Target: rhos-ontap-san-block
offloadplugin: vsphere xcopy
storagesecret.. 
storageproduct: Netapp ONTAP
Click "update mappings" to save and notice the error. 
Attached print screens of error and storage map fields 

Updating the storage map fails

I'd expect the storgemap to update/save without any issues, as I didn't have to do anything before noticing the new "eco-iscsi-ds4-vmfs5" vmware store show up under Source storage's drop-down list.

Awaiting forklift controller pod log download, I'll upload add anything I manage to find on it.