I am using Blender 2.91 and a Huion stylus. I set up a simple mesh, with a texture material, and switched to texture paint mode, using the default brush. Everything is working great when I use the mouse, but when I use the stylus it's behaving like an eraser and it's clearing whatever I touch to alpha 0. See video below.
Larger image. Left screenshot is what I get after left-clicking with the mouse and dragging; right screenshot is stylus-down and dragging.
What's going on?
Could it be a virtual/button/mapping issue of some sort? Under which condition the same thing would happen when using the mouse? Maybe if a modifier key is pressed?