It's possible that NSD traffic will go down when the "NSD via Gateway" segment is changed to another segment.
This issue occurs when you set up "NSD via Gateway" in Device -> Cloud VPN in the specific segment, delete it and replace it with another segment.
Velocloud SDWAN, VMware SDWAN, Non SD-WAN Destination (NSD) via Gateway
If you are facing this issue, you will need to contact VeloCloud support. Refer to this kb while engaging the support.