Remove button doesn't work when painting skeletal meshes

REPRO STEPS:

1. Open QAGame in the editor
2. Search for any skeletal mesh (e.g. SK_Mannequin) and drag it into the level
3. In the Modes panel change to Paint (SHIFT+2)
4. Set the Color View Mode to RGB Channels
5. Change the Paint Color to something other than white (e.g. red)
6. Paint on the skeletal mesh
7. Click the Remove button

RESULT:
The Remove button is inactive and can't be clicked, so you cannot remove paint from the skeletal mesh.