While running regular SL on my healing priest, I noticed that when dispelling Brainwash cast by the Cabal Spellbinders, it will dispel absolutely every buff the player has on them before dispelling the Brainwash. That means, any party buff that has been placed on party member must be dispelled BEFORE the Brainwash debuff will finally be removed, forcing the the use of Dispel Magic multiple times until all the target has left is Brainwash.
It's treating the controlled player literally like an enemy, but Dispel Magic, one would assume, should remove the debuff they received, which would be the Brainwash. Not sure, but don't recall having to do this during retail when trying to dispel this debuff. This also requires that buffs be reapplied every time this happens, after the fight. Couldn't find many references to this specific occurrence and how Dispel Magic worked on "mind controlled" units in retail. I could be completely wrong about this and it's working as intended, since it does indeed remove 1 beneficial buff from the enemy target (which, at the moment, is hostile).
Beside that, the Brainwashed player does nothing a majority of the time but stand there as opposed to attacking their party, or as a healer, healing/aiding the enemy. It more or less just removes them from combat, but I'm aware that any form of mind control has a ton of issues now and most don't work, like Blackheart the Inciter and player control of/by other NPCs.
Are you sure it is not a general bug with Mind Control ?
Brainwash is not supposed to be dispellable at all http://old.wowhead.com/spell=33502. Hence if you try dispelling it will dispell the buffs that the target has, as it is considered enemy target during the Brain Wash. Closing