I have used sculpt mode in Blender for years and never had this happen before: when using the mask brush, the mask is not visible. The brush is supposed to mark the area that is masked with a dark gray color. The mask brush's preview thumbnail (pictured on the left side of the screen snip) shows what the mask should look like. The sculpted object, (on the right side of the image) shows that the mask is working, but not displaying the dark gray color where the mask is applied.
Is there a way to turn on or off mask visibility or is this a bug?