REST API Map with list in response

here is my issue:

I have application with REST endpoint. This endpoint returns a list of objects ActiveAdvertisement. With this everything works fine:

I need to reorganize this response and I grouped my data by one of object fields. So now my response should return Map<String, List<ActiveAdvertisement>>.

Now my response looks like this:

Why values list inside this map are not returned as Object but this looks like Object.toString() is used there?

Map is created properly, here is screenshot from debugger:

Here is my code from Controller:

@GET
@Produces(MediaType.APPLICATION_JSON)
public Response getAdvertisements() {
  return Response.ok(advertisementAssembler.getAdvertisements())
      .header("Access-Control-Allow-Origin", "*")
      .header("Access-Control-Allow-Methods", "GET, POST, DELETE, PUT")
      .allow("OPTIONS").build();
}