I've had problems feeding too 'rich' a food (ie Blue Buffalo), and also anything with turkey in it. I also could not feed Wellness Core, but other types of the same brand seem okay.
I had one that could not eat chicken, turkey, duck, or fish - pretty much any of the 'sensitive' or 'limited' ingredient dog foods contain some of these. He could really only eat beef. So, I cooked him ground beef & rice with veggies in it for most of his life.
Hope you track down the culprit soon.
When my 2nd sheltie aged, she became quite reactive to chicken, turkey and duck, when before it wasn't an issue...in her latter years it was just easier to cook for her, than have her suffer with digestion problems. I figured she earned special treatment
. And about that time the recalls were happening with so many dog food brands.