According to Dec 1st patch notes,
- Haynar: Mana drain spells will no longer drain bard mana.
However, as a bard in a /duel with an enchanter, the enchanter was able to drain my mana. The enchanter said he was using Theft of Thought, Torment of Argli, and Lagarn's Lamentation to drain mana.
This occurred on the beta server after the dec 1 patch.