How is my IP being tracked?

Recently I downloaded some e-books from b-ok.org (formerly known as bookzz). After downloading a few books, the site notified me that: “more than 5 books have been downloaded from this IP in the last 24 hour, please sign in or register”. Confident the site was merely recognizing my Tor exit IP, I changed my VPN’s location, and rebooted Tor with a different identity. But when I returned to the site I was shocked to receive the same warning! How could this site know the same IP was attempting to download this book? I’ve tried numerous methods to alter/obfuscate my IP address (switched browsers, new identities, new guard node, router reboot, new VPN location), but this site mysteriously still knows it’s me trying to download this book.

Please, does anyone know what’s happening here? Have I committed some gigantic blunder in the construction of my network’s anonymity utilities, and been leaking my true IP address this whole time? Thank you in advance for any help.