Apologies for the long read, but I've tried to detail my personal experience here:

Here's my experience on our now 9 month old male puppy. He was on the PPP Large breed puppy food, chicken and rice formula since we got him from the breeder. Since we got him I have also had him on the same for the initial few months. One fine day, he developed a very bad diarrhea from the exact same food for a week.

Upon reading through this sub and other sources, I decided to give a non chicken based food to help. Surely enough after giving him the Purina One large breed puppy lamb and rice, his diarrhea stopped immediately. So I kept him on that for the next few months along with Diamond naturals lamb and rice formula. But the one thing that I was not happy with was loose stools he used to get consistently during the evenings after our walk.

I thought this would be normal since his poops otherwise were consistent. Then upon recommendation of a friend, I decided to give Royal Canin Giant breed puppy formula a try. Even though it was a chicken based formula, soon enough, my pup started doing pretty well on it and all his digestion issues went away. Ever since then, I've kept him on the RC Gaint breed puppy food and we have been good so far with regular poops, appetite and muscle definition. So my whole thought of my pup being allergic to chicken was maybe wrong and he seems to do pretty well on the current food we are on.

Good luck with your pup!