Xfinity Mobile Charging Me For Returned iPhone

I'm at a complete loss here. I've called Xfinity's support dozens of times and wasted tens of hours to no avail. I returned an iPhone 15 Pro to them that was delivered on September 29th. On October 19th I get an email that says:

"Unfortunately, we found functional damage to your iPhone 15 Pro, so you will be charged the full remaining balance on the device: $1099.99". 

No photos or explanation. This device was returned in the same condition I received it. They charged my card on file and support was of no help. I initiated a charge back with my bank that was won in my favor. The same day or next day Xfinity Mobile adds that amount to my balance on my account.

Now my account is overdue and I've called billing for support, they tell me it has been reversed but I'm getting alerts and automated calls telling me I'm overdue again for the same amount.

I've tried going in store but they just tell me they can't help and to call support. I am grasping at straws here. On top of this my trade-in device was lost in transit by USPS on October 9th and Xfinity Mobile cancelled my trade-in credit on November 27th.

At this point I appear to be out about $2,000 for nothing. I've gotten ZERO support even after opening multiple cases. I'm afraid if I don't pay up I'll be thrown into collections which will ding my credit. 

Case numbers for reference:

ECM0005016147

ECM0004408838

ECM0004596072