The fridge gets its cold from the freezer via a port at the top connecting the freezer to the fridge.
I would suspect the door seal. That would cause cold air to drop which might make that area colder than the top.
The leak might be at the top or bottom. It may also be that the seal is fine, but there is something holding the door slightly open. Most seals are easy to remove/replace. You may have to remove the seal to determine what might be holding the door open.