For the Cisco ACI leaf to learn individual MAC addresses from the same physical port, "MAC pinning" mode must be enabled in the vSwitch policy configuration. This mode allows each ESXi vSwitch port group to pin to a specific uplink or set of uplinks, enabling independent MAC address learning for each pinned uplink, which is essential for distinguishing multiple VMs on an ESXi host by the leaf switch. References:
Implementing Cisco Application Centric Infrastructure Official Cert Guide