I'm not vet, so take this with a handful of salt.
Kyro our 8 month cocker, and also My SIL's GSD when he was younger. has/have to occasional soft no 2. In fact Bono (GSD) has a lot of V Soft no 2's. As Kyro is getting older, and now Bono is nearly 18 mths. They are both firming up nicely.
Obviously if either of their motions as really soft, they are fasted and/or checked over. But i think it takes time for them to get used to their diets, treat chews etc.
As long as they are not too bad for too long, I believe their bodies are just settling down. Imagine how small kids are sick a fair bit, just getting used to life.
If I am miles out, then you have my permission to shoot me down in flames.