@Virgo 
Shouldn't that be possible by MAC addresses of devices?

Going to piggyback on this because I'm having the same issue.  The threads state that by having an Omada gateway it will report the traffic based on IP address, but that seems to be incorrect.  All of my traffic is routed through an SG2428P connected to an ER605 and yet I'm still showing the traffic for all of my VMs and services running on a server connected to a single port as coming from just a single arbitrary client.  The only exception to that is the actual physical server itself, which is reporting some minor traffic which I presume is correct from when I'm accessing the web interface of the server.

Is the solution to this supposed to be that this traffic has to be routed directly through the gateway itself and bypass the switch?

Regardless though, I'm unsure of why this is a problem.  As Hooch noted, this should be possible just by MAC address.  I switched from an identical network layout with Ubiquiti hardware and had no issue with proper detection of per client traffic statistics on it, so I'm unsure why this is suddenly an issue.