FYI, if someone do a SS and another does S.

If you and yet another player do plunder at same time and both try to plunder the SS

The faster one get the SS, while the slower one will get the S(or A, in theory)

Now, I'm not saying that's what happened, but you are also not showing any PROOF that it's not what have happened.

To prove that you have plundered an SS, you need to contact the player you plundered and have them confirm that

1. They were plundered by you

2. They were running SS at the time.

It would help if you take a capture of them with the gold convoy icon over there head after the fight after you see the reward is not what you expected. At the same time, I believe there is a chat log message that will help confirm the fact that you plundered that player (I never plunder, so I'm not 100% sure on this. Thou I think I've seen screenshot with it)