Welcome to All Things Plants from me, too!
Not all Draba species are evergreen. Yours might be going through its natural wintering cycle. Or it could be drying out and adapting to the paucity of water. During winters in its native habitat, it would likely be covered with snow at 12,000ft, even in Yunnan. Is this the first winter you have had it (so can you compare it to other years)? It is difficult to tell without any other information.
What region are you gardening in?
What temperatures is the draba exposed to in the alpine house?
How are these temperatures compared to outside?
Does it still receive direct sun, or is it shaded?
