DMCA notice even after using Proxy5 settings

I have been using PIA for over 5 years and have Proxy5 configured for my downloads on my BitTor##nt client. Till yesterday there were no issues but this morning, I got an email from my ISP for "DMCA Copyright Complaint Notice ID"

My PIA subscription is good till the end of 2025. Did anything change on PIA side? How did the ISP get to know I was downloading something?

My assumption was if I am using Proxy5 and downloading then my ISP gets a different IP address.