I get a large bulbus ice buildup just above the evaporator blades just under the fan motor. It takes a few weeks for the ice to buildup but eventually the ice gets so big that it pushes the rear of the freezer out and makes a noise.

I have attached two pictures (one before, one after). Why is this happening? The schematic diagram doesn't indicate this part. What needs to be replaced?

I assume the heater is functioning since the blades are frost free and the sides show some signs of melting / burn.

Any thoughts? See attached images.