Nested serializer names disappear in drf-spectacular swagger-ui schema view when read_only or allow_null is set?

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I am using the drf-spectacular library together with Swagger-UI to document my Django Rest serializers (schemas).

What I have noticed is that if I set the read_only=True or allow_null=True attribute of any nested serializer fields, the name of that nested serializer disappears from the schema view in Swagger-UI. If I remove those attributes or set them to False, the names re-appear. Not sure why this is happening?

class MySerializerSerializer(serializers.ModelSerializer):

    read_only_field = MyReadOnlySerializerSerializer(read_only=True) #--> 'MyReadOnlySerializer' name is not shown
    null_field = MyNullSerializerSerializer(read_only=False, allow_null=True) #--> 'MyNullSerializer' name is not shown
    normal_field = MyOtherSerializerSerializer(read_only=False) #--> 'MyOtherSerializer" name is shown

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