Hmm, now my Plasma count is stuck, too. However, I just got the Polaron accolade.

I suspect, it has to do with the weapons you are using - I was using some Mk VI Plasma Whites while observing the counter rise a couple days back - this merits more testing.

Edit: It seems like not the weapon, but rather the locale is the buggy factor. In B'Tran it doesn't seem to work for me. While I was killing Gorn on Kassae, it does however. It also does in the Big Dig (Quarn can also confirm this).
In any case, it is a more obscure bug it seems.