Which version of NSX you are running.
I do had faced this problem earlier, when trying via GUI it shows that packets are not received.
But via CLI observed no packet loss. It was looking more like the GUI error
Can you try with the size of 1600 via cli & check the output ?
The versión is 6.3.5
I chech to 1600 and the result is OK by cli.
It should be issue related to GUI which i have observed in NSX version 6.2.2
Since the CLI output is as expected, its fine. It will not be an issue.