This could actually be a bug in the host, if the host is running ESXi:
ESX Driver Problem
I have the same issue in my network on Nexus 9396PX switches, though FEXes are not involved. 2344-byte pings succeed. 2345-byte pings fail. But the host that's pinging is running ESXi, so we are looking into drivers to see if that will resolve it.
We have observed no problems when the source and destination of the pings are other Nexus switches. 9000-byte pings with the DF bit set succeed every time.
