Puppy food opinions

Hey. Our newf is nearing 4 months. We’ve had her at home since 8 weeks. We started with feeding her the same as the breeder - purina pro chicken and rice sport 26/16. I always thought her stool was on the softer side but it was well formed and at normal intervals. We incremented as she got older and she might got gotten into something which caused a case of diarrhea. Her stool was negative and the vet put her on probiotics and a chicken and rice diet for a week. It got back to her normal but still not as firm as I would expect. Coming off of that we switched food to giant breed puppy royal CANIN at the vets suggestion. Increasing to even 1 cup per meal has made her diarrhea come back and it seems correlated directly to this food. I’m unsure where to go from here and would love suggestions.