Serializer Field's "source" Changing Serialized Data Key

[kolanos]

I have the following:

class Difficulty(IntegerChoices):
    EASY = 0, "easy"
    MEDIUM = 1, "medium"
    HARD = 2, "hard"

class MyModel(models.Model):
    difficulty = models.SmallIntegerField(choices=Difficulty.choices)

class MySerializer(serializers.ModelSerializeR):
    difficulty = serializers.ChoiceField(choices=Difficulty.labels, source="get_difficulty_display")

    class Meta:
        fields= ["difficulty"]
        model = MyModel

    def create(self, validated_data):
        print(validated_data)

Why does this give me:

{"get_difficulty_display": "easy"}

instead of:

{"difficulty": "easy"}

And if this is not a bug, how do I get the latter behavior?

And yes, I know I should just use TextChoices, that decision predates me.