Hi Mattunderwater,
It is understandable to be confused. Before there were specific diets for large breed puppies it was a common recommendation to switch them to an adult food before fully grown.
Now that large breed puppy foods are available, veterinary nutritionists recommended that a puppy food appropriate for large breed growth food be fed until 80% mature, ~18 months-2 years depending on the breed.
https://veterinarypartner.vin.com/default.aspx?pid=19239&id=5985068
I don’t know anything about the brand you are using to know how their foods are made, some “adult” foods meet requirements for growth, but the information given to you by pet shop and trainer is not current.
If the food you are using is not made for growth/ juniors than I’d consider switching to one that is appropriately formulated for the growth of large breed puppies (not all diets labeled for large breed puppies are appropriately formulated) and feeding it in amounts to maintain good body score.
