Another thing I’ve noticed:
It’s affecting my ability to use InsertMesh and curve brushes. When I try to use either of those, the mask appears, and I can’t edit the new object. Zbrush is considering my main model and my new addition to be joined. 
EDIT:
Ok, I figured out that when I use either tool, if I then click "Clear’ mask, the mask goes away. I don’t know why that mask is stored in memory at all. Anyway, the problem I have no is, once I clear the mask, Zbrush treats the new object as if its welded already to the main model, so I can’t move, rotate or scale it.
Any ideas?