I can reproduce the problem. I first assumed that it was a bad exit node. Tor doesn't add an additional encryption layer to the last leg between exit node and a regular (non-.onion) website. So exit nodes can eavesdrop or perform a man-in-the-middle attack if they want to and that connection isn't encrypted in some other way. However, that page appears in the HTTPS version, and the certificate checks out. It's the same I get when I access the website normally. It's a certificate by Let's Encrypt with the following SHA-256 fingerprint:
That can only mean one of two things:
- It's legit. This is actually the page the asstr.org webmaster wants you to see when you access their website with TOR.
- The admin of the exit node somehow obtained the private key from asstr.org, allowing them to impersonate them.