Elphidran's drop increase skill does not seem to work on boss drops. Is this a known phenomenon?

I fought the Dandilord boss ten times with a four-star condensed Elphidran, and ten times without. I got an average of 71.5 holy water per fight with Elphidran in the party, and 66.5 without it. If the skill was applied to the drops, it would have resulted in an \~80% increase, for an average of 119.7, but I only saw a \~7.5% increase, which can probably be chalked up to the randomness of the small sample size. Is it supposed to not work on boss drops?