I would guess that it is a manufacturing problem with the physical TFT. After all, it must be pretty difficult to get the variable transmissive response for each primary colour.
The integrated circuit technology tends to either pass or fail. It is when you marry the i.c. electrical signals to the "big" LCD array that must be difficult to align.
Have you tried my sketch? Do you see two distinct sets of green bands?
Do you see more than 4 levels of green?
I can understand "several" bad green shields. It seems unlikely that "all" shields with the "flex at bottom right hand edge" style would fail. The Chinese would have noticed !!!
David.