This is a very late reply, but we just experienced exactly the same symptoms with the same switch model etc. As it happens the switch model was irrelevant.
In our case, it turned out that the management portgroup had different load-balancing settings to the vSwitch that it was on, so although the vSwitch was set to IP Hash, the portgroup was set to override that and use Port ID. No idea if that was the case for you but I thought it worth posting in case anyone else finds this question and no previous answer.