Weight loss in a growing puppy is not normal. For the most part the weight especially in a large dog will gradually increase until growth stops anywhere from 9 months to 1.5 years. For some of the larger breeds the trick is to keep them from growing too fast and developing hip problems.

In a young dog weight loss can occur most commonly due to parasites. But these will usually lead to other signs with the intestinal type causing GI problems, while others that may reside in the lungs causing respiratory (e.g. cough/gaging) problems.

Other health causes could be due to the onset of Juvenile Diabetes, where the dog may have a very healthy appetite, along with excessive thirst. This can be picked up on a blood panel work up. Or there could be disgestive enzyme problems such as pancreatic insufficiency, thus the food passes through before it can be digested totally. This can also be picked up on with blood testing. Food allergies can sometimes influence absorption of nutrients from the food.

Finally there is the obvious that the dog isn't getting enough food or a high enough protein/fat content to put on weight. A puppy dog which is underweight should indeed be getting a performance formula which means ~30% protein/15% fat. I presently have my thin puppy of 6 months on a Puppy performance food from Pro Pet (owned by Carnation). It consists of 32% protein/18% fat. Although she is thin, she is not as underweight as yours. If the health checks out at the Vet, and there is no harm in getting a second opinion from another Vet, then give her free choice to her food.

P.S. Stay away from the raw food, especially the raw eggs and potential salmonella. Most of the high performance dog brand foods are much more balanced in nutrition (for a dog not human) than anything you can cook up.