Cascading Wings: Do they really increase lift by 300%?
The software appears to be taking the net plan area or shadow area of the overlapping stack as its area, but is adding the individual lifts of each wing. If you add the individual areas too then the calculation should show a fall in Clmax due to interplane losses.
I'd call it a bug, however I do not know the software so perhaps it is a "feature". It depends on whether you regard your stack as one wing or four.