One more thing, this is not just for print service. It happens for any types of service, for example geometry service. If you are trying to reproject a geometry through a geometry service, and the domain of the geometry service has not been accessed before, the same situation will happen. It will send a call to rest info to decide if CORS is supported or not, but the first request to the geometry service will go through proxy since it doesn't know the result yet from the rest info call.