“Heroes of Might and Magic V: Tribes of the East”

Gentlemen, I was recently somewhat dismayed by the fact that the supposed magic immunity of black dragons doesn't actually work.
Or rather, the issue lies with the Staff of Sar-Is, which, according to the description, should reduce resistance to magic, but in reality, it bypasses the immunity of black dragons, even though the description states that it shouldn't.
Is this a bug in the staff's code or an error in the description?