Complex properties are formatted as application/xml. Only entries and feeds (entities and collection of entities) are formatted as application/atom+xml. So, the behavior that you are seeing is correct.
You can get the behavior that you want by putting application/atom+xml and application/xml in the accept header like, "application/atom+xml,application/xml"
You can get the behavior that you want by putting application/atom+xml and application/xml in the accept header like, "application/atom+xml,application/xml"