I have actually had this problem and repaired it. It is your defrost condensate check valve.
It gets gummed up, it won't drain and the the freezer jams up with frost. Simple fix/replacement.
This video explains it for one model, the principle remains the same for yours. Just remove and replace for less than 15.00. It certainly is worth fixing and it is unlikely it is low refrigerant or some other complex repair.
Fixing a Clogged Defrost Drain Check Valve in a Whirlpool-built Bottom-Mount Refrigerator - YouTube
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