Green is a primary in the computer (RGB). Whereas green is a secondary color in the real world (RYB). The defining characteristic of primary colors is they cannot be mixed from other colors... so the behavior of not mixing to green is predicatable and accurate for a RGB color mixing space.
If you would like green to be a mixable color then what we would need is a RYB painting colorspace in Rebelle. (I would support that request BTW).