I create a mask, hide it by sliding it to the right, and then paint over a large area using a "live brush". If I then delete the mask, I find that the whole area I painted shows paint. In other words, the mask did not mask the area. I think of the mask as the same as "masking fluid" in real watercolors -- it should prevent any paint from leaking under the masked area. Do I misunderstand the purposes of masking, or is this a bug?
In this case, you're misunderstanding how masks work. A layer mask is a way of dynamically hidingnoarts of a layer. Black conceals, and white reveals, showing the selected portions of the underlying layer. A mask can be painted on to hide or reveal additional parts of a layer after the fact.
If you're trying to restrict the flow of live paint, you need to paint within an active selection. Masks only hide areas, they don't prevent you from painting anywhere.