additionalProperties not correctly handled

[bradzacher]

I'm trying to generate types for the SARIF JSON Schema spec - https://json.schemastore.org/sarif-2.1.0.json

In particular they have defined:

{
  "originalUriBaseIds": {
    "description": "The artifact location specified by each uriBaseId symbol on the machine where the tool originally ran.",
    "type": "object",
    "additionalProperties": {
      "$ref": "#/definitions/artifactLocation"
    }
  },
}

I would have expected that this would generate a class with additionalProperties as mentioned in the wiki, like:

public class OriginalUriBaseIds {
  private java.util.Map<String, ArtifactLocation> additionalProperties = new java.util.HashMap<String, ArtifactLocation>();
  
  // ...
}

However instead this generates a class without additionalProperties:

@JsonInclude(JsonInclude.Include.NON_DEFAULT)
@JsonPropertyOrder({

})
public class OriginalUriBaseIds {


    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append(OriginalUriBaseIds.class.getName()).append('@').append(Integer.toHexString(System.identityHashCode(this))).append('[');
        if (sb.charAt((sb.length()- 1)) == ',') {
            sb.setCharAt((sb.length()- 1), ']');
        } else {
            sb.append(']');
        }
        return sb.toString();
    }

    @Override
    public int hashCode() {
        int result = 1;
        return result;
    }

    @Override
    public boolean equals(Object other) {
        if (other == this) {
            return true;
        }
        if ((other instanceof OriginalUriBaseIds) == false) {
            return false;
        }
        OriginalUriBaseIds rhs = ((OriginalUriBaseIds) other);
        return true;
    }

}

This of course causes a crash when deserialising an object that uses the map

Is this a bug in the codegen, or am I "holding it wrong"?