This is because the triangle that is being displayed on the screen is different from the one that is being picked by the CameraPick.
The CameraPick sees one triangle and the Blitz3D render engine displays another (camera range doesn't seem to make a difference).
Decorator then takes the picked triangle and tries to move it to another surface. But the one it picked is not the one you see. So it picks the same one and moves it again and so on.
Yes, that's exactly what happens. It seems you cannot paint a triangle, instead it paints a triangle that is hidden behind it's clone.
I don't use any other tools, the extra tris where generated by Decorator somehow. Probably it happens in some rare exceptional case only, but if there's once a clone, they "multiply" quickly.