The reason is that the ProcessDefinitionEntityImpl (which is the implementation of `ProcessDefinition) is not designed for direct serialization. Jackson uses reflection to perform the serialization, so it will serialize things that are not in the interface.
Also in general it is never good to serialize interfaces where you don’t know the actual implementation, usually over REST you want to have a defined API and what it would be returned. Therefore, defining your own DTO or using the ProcessDefinitionResponse is the way to go (in my opinion).