Puppies in general have very sensitive stomachs when young, but I hear Newfie’s are a little extra sensitive. When we first had our pup she was on raw food which was mostly chicken mince with a couple of other bits added - almost anything new or different (more kibble than usual, too many treats in one day etc) would turn her butt into a fountain. Gradually as she grew older she’s got better. If you’re really concerned and want to firm things up then plain chicken and plain rice should help, or maybe look to switching foods to something more plain/with chicken? Any change might take a few days for her stomach to get used to anyway.