How are you testing Drain on undead? If you're using one of the Drain weapons you should note that they're all set to do 4x damage to undead, that's what's causing the extra damage. The drain element just tells the game to make that weapon act like drain, an enemy can't be set to be weak against it.
Edit: Actually, they may be able to. The enemies can be weak to Fire, Ice, Lightning, Dark, Holy, Aerial, Unknown, and Weak. The Unknown happens to be in the same place Drain is in the list of weapon elements, so it's possible that it is indeed a Drain weakness that they never bothered giving to any monsters. I'm going to test that right now.
Edit 2: It definitely
is a drain weakness, Kain was doing between 150-300 damage on an unchanged FlameDog with his Drain Lance. After giving it the unknown weakness he was doing between 500-1000.
