I didn't have any difficulty getting the rare treasures. It's the common treasure I can't get, and the treasure finder won't help with that.
Ah, so it is a greater alchemite. I noticed similar behavior with the common treasure greater alchemite in the dark cave. It seems that the inherent rarity of the greater alchemite as common treasure is so high that it's considerably faster to farm them as rare treasure with the treasure finder.
edit: I just got five of them in a row. Weird.