The drain is on top of a buildcraft tank which has a extraction pipe + autarchic gate pumping liquid XP into the XP bottler.

Once the buildcraft tank has filled, the player can stand on the drain and the bottler will continue to fill bottles without draining any XP from the player.

Forge: 9.11.1.953
OpenBlocks: 1.2.2
BC: 4.1.2

Curious. I'm assuming it puts your XP in to the tank, but as the tank is full, it fails, or at least the amount is so small that it looks like it fails as far as number rounding goes, causing 0 levels to be drained from the player.

My guess is that this:
int xpDrained = EnchantmentUtils.liquidToXPRatio(filled);

rounds to 0, consuming 0 levels but filling the tank.

Not tested, that's your job :P
Let me know if the issue still exists (: