i have this issue few months back and thought there must be something wrong with the fridge or there are gaps somewhere that allow fruit flies to go inside my freezer. Turns out its not the fridge, I have a pack of expired raisins hidden somewhere on the table top next to the fridge that was drawing the flies in, and they manage to find their way into my freezer.

After throwing the icky pack of raisins away, flies are gone now.