Here is another test worth trying. Switch off the Camera then dismount the lens for 30 seconds then reattach the lens and leave the camera switch off over night to see if there is any power drain.
I'm suggesting this to see if physically removing the lens after powering off the camera will break the electronic circiut that is causing the drain and if it remains so even once the lens has been reattached and the camera being left off for a long period.
The other thing is that maybe this problem is only on your copy of this lens.