Colonoscopy billed as 'diagnostic' because they found a polyp - now I owe $2,800 I thought was covered

I went in for a routine screening colonoscopy. My doctor ordered it, i'm 52, totally standard preventive care. My insurance covers preventive colonoscopies at 100% - no cost sharing. I got the procedure, everything seemed fine. Then the EOB showed up and I almost fell over. The entire thing got billed as a diagnostic procedure because the gastroenterologist removed a small polyp during the scope. My cost share: $2,800.

I called the insurer. They said the moment any tissue is removed or biopsied during a colonoscopy, the billing code changes from preventive to diagnostic, and diagnostic procedures are subject to my deductible. I had no idea. My GI never mentioned this. The whole point of a colonoscopy is to find and remove polyps - that's what makes it preventive care. How is it "diagnostic" if there was nothing to diagnose beforehand?

I filed an appeal and they denied it. Their position is that the coding is correct under ICD billing guidelines. I'm honestly at a loss. This feels like a billing trap. Has anyone gotten this overturned? Is there any argument that actually works here or am I just stuck paying this?