Quick question - are you seeing same behavior from Nexus toward EX2300, that is only one link is carrying traffic? Or question might be are you only seeing this EX2300 toward Nexus, or in both directions.
It is very likely that EX2300 may not have support for user configuration of hash algorthm. Is the traffic L3, that is would source/destination MAC pair likely be the same for all traffic? I would think traffic is L2, and therefore should maybe have a single destination MAC (Nexus routing MAC) with multiple source MAC.