This has been a long-time vexing problem in some LG refrigerator models. You'll also see this on Kenmore and GE refrigerators that were built by LG. The unit defrosts properly but builds up rime ice inside the freezer looking like there's an air leak, like from a bad door seal. But the door seals are good and no source of air leak can be found. Brother Durham found this tech memo that explains it and offers the cure: a redesigned evaporator fan motor.
This service bulletin covers your model... it may be of assistance http://appliantology.org/files/file/915-service-bulletin-lg-fridge-ice-in-freezer-compartment/
new grillefan has increased insulation.
Here's the upgraded evaporator fan: http://www.repaircli...er=795.78092900