[BUG] [JAVA] "additionalProperties" does not generate a HashMap Model, openapi: 3.0.2

Description

I am using "additionalProperties" in my yaml file as below.

openapi: 3.0.2
....
components:
  schemas:
    Messages:
      type: object
      additionalProperties:
        $ref: '#/components/schemas/Message'
    Message:
      type: object
      properties:
        code:
          type: integer
        text:
          type: string

But when running mvn clean package, no class is being created for Messages. I was expecting to see a class like :

public class Messages extends HashMap<String, Object> {
......
}

in org.openapitools.client.model.Messages the same as other classes generated based on the yaml file. I tried additionalProperties: true and additionalProperties: {} but none of them worked. It seems like a bug

my pom.xml for openapi-generator and swagger versions

<dependency>
  <groupId>io.swagger</groupId>
  <artifactId>swagger-annotations</artifactId>
  <version>1.6.1</version>
</dependency>
....
<plugin>
  <groupId>org.openapitools</groupId>
  <artifactId>openapi-generator-maven-plugin</artifactId>
  <version>4.3.1</version>
  <executions>
    <execution>
      <goals>
        <goal>generate</goal>
      </goals>
        ...
    </execution>
  </executions>
</plugin>

Command line used for generation

mvn clean package

Start with the OpenAPI schema in the issue and the openapi-generator Maven plugin configuration in pom.xml, then run mvn clean package to inspect the generated Java models. Reproduce the missing Messages model and identify the generator path handling additionalProperties; done means a Messages model is generated with the expected map value type and relevant generation coverage exists.