Healthy Paws covered years of treatment for one dog, then denied a nose biopsy for my new dog

My previous dog had an autoimmune disease, and Healthy Paws was terrific about ongoing medicine and injections for years. That is why I chose them again for our new dog. This time they denied a $1,300 biopsy on a nose spot that turned out not to be cancer. They connected it to a notation about dry skin on his nose 11 days after coverage started. Our vet wrote an appeal saying the biopsy was for a separate concern, but the insurer still would not cover it. I am trying to understand where one symptom ends and an unrelated issue begins.