Roper--ice buildup on one freezer shelf

Question

I have a ROPER upright FREEZER model # RV1299REW00. It has three shelves. The top shelf constantly experiences severe ice buildup to the point where you can't put anything on it. I have to constantly defrost it once the ice buildup gets 4-8 inches deep. The lower 2 shelves remain fine. I know the drain tube and passageway on the bottom is clear from when I defrost it. The seal looks fine all the way around. The manual referenced refrigerated shelves, but it was for multiple models and I don't know for sure if my model has those or if it is relevant.