I have apparently found a bug. When I explore the Dirn Kibhar Mine with the mithril treasure finder enabled, I never get greater alchemite or plumberries. I only get tons of blissful herbs and the greatest alchemite, which still drops when the meter gets full.
Frequency appears to drop back to normal when I unequip the treasure finder.
It may affect other locations, too, but I've only noticed it here so far.
Yes, I am aware of what happens when there are no more special items. The problem is that a couple of the remaining "infinite" items never appear at all when the finder is enabled, especially the greater alchemite, the most valuable of them (I don't care about the plumberries, they're available everywhere).
This is not the expected or desired behavior. Using the finder should at the very least not keep you from getting something.