We are also experiencing the same issue when disconnecting / reconnecting vNIC on VM's hosted on ESXi 6.7u3 hosts. Here's the VMware KB on this:
Workarounds:
Use anything but the H5 client to reconnect the vNIC (Flash/Flex client, powershell, etc)
Switch the vNIC portgroup to any other portgroup, enable the vNIC, then switch back to the original portgroup
