During Hyper-V backup, we execute VSS API call DoSnapshotSet, as soon as it completes, we execute ImportSnapshot VSS API call, without any delay between two API calls.
Apparently Dell VSS hardware provider continues to write some data to VSS snapshot after execution of DoSnapshotSet method(after returning S_OK code), and if we try to immediately import VSS snapshot, we could backup inconsistent data from that VSS snapshot.