The Veeam backup job fails but the SAN snapshot on HP Store Virtual is still created and left behind.
On the proxy you may see in the event logs 2 events, one after another: first VSS event 8229 about VSS writer failure and then event 284 from HP StoreVirtual VSS Provider about successful snapshot creation.
This happens due to incorrect behavior of HP StoreVirtual VSS Provider: in some cases it still creates and keeps a snapshot even after VSS writer failure.
Veeam always creates snapshots with the default attribute "auto release" which means that shadow copy is automatically deleted when the shadow copy requester process ends. But neither VSS subsystem, nor Veeam would know if the snapshot was created by provider after the VSS writer failure, because we send all requests about snapshot status to VSS writer. So this is impossible to track from Veeam’s side.
If you see such behavior, please open a case with HP support.