Default value not honored when deserializing a JSON avro event using Apache AVRO Java 1.11.1

Question

I have a very large AVRO schema with this record as one of its types:

 {
    "namespace": "com.pingcap.simple.avro",
    "name": "DataType",
    "type": "record",
    "docs": "each column's mysql type information",
    "fields": [
      {
        "name": "mysqlType",
        "type": "string"
      },
      {
        "name": "charset",
        "type": "string"
      },
      {
        "name": "collate",
        "type": "string"
      },
      {
        "name": "length",
        "type": "int"
      },
      {
        "name": "decimal",
        "type": ["null", "int"],
        "default": null
      },
      {
        "name": "elements",
        "type": [
          "null",
          {
            "type": "array",
            "items": "string"
          }
        ],
        "default": null
      },
      {
        "name": "unsigned",
        "type": ["null", "boolean"],
        "default": null
      },
      {
        "name": "zerofill",
        "type": ["null", "boolean"],
        "default": null
      }
    ]
  }

What I'm seeing when I try to deserialize this using the following code:

DatumReader<DDL> reader = new SpecificDatumReader<>(DDL.class);
Decoder decoder = DecoderFactory.get().jsonDecoder(DDL.getClassSchema(), new String(data));
return reader.read(null, decoder);

Is that the fields marked as default the MUST be in the data otherwise I get an exception. For instance: org.apache.avro.AvroTypeException: Expected field name not found: decimal

Here is that specific part of the data:

"dataType": {
    "mysqlType": "int",
    "charset": "binary",
    "collate": "binary",
    "length": 11,
    "decimal": null,
    "elements": null,
    "unsigned": null,
    "zerofill": null
  }

If I remove any of those nullable fields, I get the exception.

I am using the tools to generate the classes of the schema:

java -jar ./avro-tools-1.11.1.jar compile schema ./schema.json src/main/java/

Anyone know why the defaults are not being picked up and need to be explicitly encoded?