RESTEasy does not properly determine the media type for response. It seems to skip, or at least partially skip, the last step of part 2.

Else set P={V(writers)} where writers is the set of MessageBodyWriter that support the class of the returned entity object.

What we skip is retrieving the @Produces for the MessageBodyWriter which can support the object type.

See the long discussion in the linked forum post for more details.