Could someone please tell me where or how to find Grade 5 Carbonized Matter?

My gathering log says I have acquired it, but it was long ago and probably by chance since I've never aimed to gather it until now.

I've managed to use Luck of the Pioneer on several nodes in Voor Sian Siran, the Smoldering Wastes, and the Chocobo Forest, but every time I use it I get a message saying that I have already uncovered all the node's hidden items.

Am I looking in the wrong places even though my log says it should be there? Or am I doing something else wrong?