It sounds like there is something in the environment that is forcing the DC (device context) to render on another Window handle. This can happen, but default behavior should not cause this. The only time I generally see stuff like this is when there is something fired within an event, thread, etc. that forces the handle back to some other context. I created a quick sample and used the multi-column approach as this performs custom rendering and should be more apt to reproduce the problem. Try the sample attached to see if you have the same problem as I could not reproduce your problem here.
If you are setting the DroppedDown property through an event, you might consider (just for testing) to set a timer then set the DroppedDown property through the timer to see if the probem goes away. If so, then you know the culprit and can go from there.