[BUG] Object with additionalProperties detected as "free-form"

Description

I have a type Money and a type MoneyTuple, which simply has additionalProperties: $ref: '#/Money'. Despite that the properties are all typed, the Java generator decides that this is a "free-form object" and doesn't output a DTO for it. Other types ref'ing MoneyTuple then fail.

The equivalent code NumberTuple where the values are numbers works as expected.

openapi-generator version

I'm using OpenAPI Generator 4.3.1 from the Maven plugin.

OpenAPI declaration file content or url

components:
  schemas:
    Money:
      type: object
      required: [currency, amount]
      properties:
        currency:
          type: string
        amount:
          type: string

    MoneyTuple: # ignored as "free-form object"
      type: object
      additionalProperties:
        $ref: '#/components/schemas/Money'

    MoneyTupleSeries: # fails on compilation because MoneyTuple isn't generated
      type: array
      items:
        type: object
        required: [label, values]
        properties:
          label:
            type: string
          values:
            $ref: '#/components/schemas/MoneyTuple'

    NumericTuple: # extends HashMap<String, Double>
      type: object
      additionalProperties:
        type: number
        format: double

Generation Details

I am using the Maven plugin with <generateAliasAsModel>true</generateAliasAsModel>.

Start with the Java generator's handling of OpenAPI schemas using additionalProperties, and reproduce the supplied Money, MoneyTuple, MoneyTupleSeries, and NumericTuple declaration through the Maven plugin with generateAliasAsModel enabled. Done means MoneyTuple is generated as a typed map or DTO as appropriate, and MoneyTupleSeries compiles without the missing-model failure.