In some configurations, when a FortiGate with NP7 or NP6 processors is operating with virtual clustering enabled, traffic cannot be offloaded by the NP7 or NP6 processors if the MAC address of the VLAN interface accepting the traffic is different from the MAC address of the physical interface that the VLAN interface has been added to. If you are running a configuration like this, traffic from the VLAN interface can be dropped by the NP7 or NP6 processors. If you notice traffic being dropped, you can disable NP offloading in the firewall policy that accepts the traffic to resolve the issue.
NP7 and NP6 offloading can still work in some network configurations when a VLAN and its physical interface have different MAC addresses. For example, offloading can still work as long as other network devices learn the FortiGate's MAC addresses from ARP. As well, offloading can work if the reply traffic destination MAC is the same as the MAC of the underlying interface.