The interdiction mini-game has been made a lot easier than it used to be, and are now quite easy to win. Avoiding an interdiction this way stops the NPC, the old submit, boost, boost, jump, still works of course, but the NPC will just chase after you again.
As for dropping cargo, there are two options, Jettison and Abandon. Abandoning it makes it legal for people to pick it up, so maybe you need to do that, jettisoning it makes it illegal for people to pick it up, so maybe the pirate doesn't like that. I'm not sure on this however as I've never given cargo to a pirate. Note that you can not abandon mission cargo (i.e. stuff given to you to deliver).
Of course being ED, it could well just be a bug, if it is a bug then file a support ticket and you'll get your stuff back.
Please make sure you have read the Mobius GroupPolicy