Hello Brian,
CoPP control plane policing may play a role here.
Another point is where in a VSS the fragmentation happens. if the packet is received on a L2 port of the standby chassis it should go via the VSL link to the master supervisor.
the VSL link has probably an MTU able to accomodate frames of size 9216 bytes + VSS overhead of the protocol.
This might explain the different behaviour.
However, sending so big packets that need to be fragmented by first hop network device is not common and it is not best practice.
The application tester/developer can change the size in use and he/she has already done it.
>> Other than the testers complaining, we couldn't detect any issues.
Application developers/ testes always complain of network they think of networks with infinite bandwidth, very low delay and infinite MTU but reality is different.
Hope to help
Giuseppe