Guild Wars 2

I just finished the recent part of the Living Story with my second character and I noticed that I did get a second exemplar of every part needed for the vine backpiece… except for the Pile of Phantasmal Residue. Is this a bug or some sort of intentional taunt?

It’s either. The Residue originally gets awarded upon completion of the story through the achievement. You can only get that once.
If you finish Foefire Cleansing and then talk to Rox after receiving your rewards (or when you should, depending on if it’s a replay) she’ll give you Residue. I tried to throw it away and talked to her again without leaving the instance and she gave me another one; so you can either replay the instance for as often as you need Residue or – if that works – get as much Residue as you want in one run through talking to Rox.

Funny thing about the Residue (I got two on my first play through – one from the finished-story-splash, the other from Rox), the uncharged Residue doesn’t stack, while the charged Residue does.