I am using swagger-parser 2.1.25 and when I parse an OpenAPI document openapi-properties-additionalProperties.json using these ParseOptions:

ParseOptions parseOptions = new ParseOptions();
parseOptions.setResolve(true);
parseOptions.setResolveRequestBody(true);
parseOptions.setResolveFully(true);

OpenAPI openAPI = new OpenAPIV3Parser().read("src/main/resources/openapi-properties-additionalProperties.json", null,
    parseOptions);

It works perfect without any issues. swagger-parser can parse both properties and additionalProperties. However, when I change the requestBody structure from

"requestBody": {
  "description": "Update an existent pet in the store",
  "content": {
    "application/json": {
      "schema": {
        "$ref": "#/components/schemas/Pet"
      }
    }
  }
}

to

"requestBody": {
  "description": "Update an existent pet in the store",
  "content": {
    "application/json": {
      "schema": {
        "allOf": [
          {
            "$ref": "#/components/schemas/Pet"
          }
        ]
      }
    }
  }
}

as in openapi-allOf-properties-additionalProperties.json, where Pet schema has both properties and additionalProperties like this in both documents:

"Pet": {
  "type": "object",
  "properties": {
    "id": {
      "type": "integer",
      "format": "int64",
      "example": 10
    },
    "name": {
      "type": "string",
      "example": "doggie"
    }
  },
  "additionalProperties": {
    "type": "string"
  }
}

Resolved Schema has additionalProperties as null, where properties is resolved correctly. I believe that ResolverFully.aggregateSchemaCombinators method does not handle additionalProperties like it handles properties. In order to solve this issue, I added the following code before the line Map<String, Schema> properties = resolved.getProperties(); in aggregateSchemaCombinators method, knowing that it is not the perfect fix that handles all of the scenarios:

targetSchema.setAdditionalProperties(resolved.getAdditionalProperties());

It solved my issue, but probably it may not work in a more complex structure. Do you have any suggestions?

When I try to test these two OpenAPI documents from https://editor.swagger.io/ I can see both properties and additionalProperties in both files are resolved correctly. Example Value:

{
  "id": 10,
  "name": "doggie",
  "additionalProp1": "string",
  "additionalProp2": "string",
  "additionalProp3": "string"
}