I have an Irish Wolfhound, adopted at an older age, and she can’t tolerate a lot of foods. Lots of IWs are the same, I guess, but her symptoms sounded like your pup’s. Have had her on Dick Van Patten’s Natural Choice, and now on Tractor Supply’s 4Health. Although I’d normally say follow your breeder’s recommendations for your dog, if this keeps up, it may be the new food. Believe with my girl, it’s best to avoid those foods with wheat up front.
Chicken/rice is a typical solution for the runs, but you’ll know if it’s the food when you add it back. And your dog may be allergic/sensitive to even some of the most expensive foods - it’s the ingredients you may have to keep an eye out for.