I'm going to take a shot in the dark that it's supposed to stay open. The butterfly was installed on your car primarily to allow the DSC to function (also as a failsafe, I think).
Remember the valvetronic controls your throttle on that car. I would imagine it stays open to reduce pumping losses (which is one of the original premises of Valvetronic).
That's my guess...
'09 335d Alpine White/Beige Dakota, Sport, Premium, Cold Weather, iDrive, PDC, CA, iPod, Premium Sound
Retired: '93 325i |'99 Jeep Wrangler Sport |'06 330i Sport |'08 335i Sport |'08 328i Sport