Frost? Like just a freezer or you mean fridge/freezer?
My experience is frost comes from humidity from opening too long or location (I have a chest freezer in garage and it's always frosting up after a few months due to kids opening and "looking" for whatever and it's 95 degrees in garage)
Do you have anything on top the freezer (on the outside like microwave or other stuff). Sound like possibly the insulation on top not doing best job allowing warm air to intrude
I would think it's pulling in humid air from somewhere. Does it have a fan?