Could the tablet be controlling the pressure even when not in use.
It shouldn’t!
The Masking color is an Overlay and it is set to 0.75 as default…try changing that..also make sure the Strength of the Mask Brush is 1:
If you have a lot of topology detail (read many Vertices), then the Smooth Brush won”t affect the Mesh so much, but if you try to smooth the default Cube (with only 8 Verts), the smoothing will be “catastrophical”…