How is load balancing on vswitch configured ?
what is your North bound switch configuration; e.g. 2 separate switches, connecting to either FI ? and do the switches have a trunk with the proper Vlans of the UCS domain. (maybe you have a vpc)
Could be, that on the same vlan, egress traffic of VM-1 is in fabric A, and ingress to VM-2 is on fabric B; therefore although this is L2 end to end (within in Vlan), the traffic has to exit the UCS domain, and enter the domain on the other fabric.