Hi,
Try setting lower values in the "Threshold" parameter.
In this mode, the map defines not only the plant type (by the Color ID), but also the presence or absence of items. Forest consideres "dark" pixels as gaps in the distribution, and the "darkness" of a pixel is evaluated by the threshold parameter. In terms of luminance, red color is darker than green, so at 50% of threshold it's considered as a black pixel.