portcheck.transmissionbt.com fails over IPv6

Transmission port check bug

$ curl -6 -v https://portcheck.transmissionbt.com/51413
*   Trying 2001:41d0:c:5ac:5::1...
* TCP_NODELAY set
* Connected to portcheck.transmissionbt.com (2001:41d0:c:5ac:5::1) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: CN=build.transmissionbt.com
*  start date: Sep  6 11:00:59 2018 GMT
*  expire date: Dec  5 11:00:59 2018 GMT
*  subjectAltName: host "portcheck.transmissionbt.com" matched cert's "portcheck.transmissionbt.com"
*  issuer: C=US; O=Let's Encrypt; CN=Let's Encrypt Authority X3
*  SSL certificate verify ok.
> GET /51413 HTTP/1.1
> Host: portcheck.transmissionbt.com
> User-Agent: curl/7.58.0
> Accept: */*
> 
< HTTP/1.1 400 Bad Request
< Server: nginx/1.10.3 (Ubuntu)
< Date: Sat, 03 Nov 2018 17:49:01 GMT
< Content-Type: text/html; charset=ISO-8859-1
< Transfer-Encoding: chunked
< Connection: keep-alive
< 
* Connection #0 to host portcheck.transmissionbt.com left intact

$ curl -4 -v https://portcheck.transmissionbt.com/51413
*   Trying 87.98.162.88...
* TCP_NODELAY set
* Connected to portcheck.transmissionbt.com (87.98.162.88) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: CN=build.transmissionbt.com
*  start date: Sep  6 11:00:59 2018 GMT
*  expire date: Dec  5 11:00:59 2018 GMT
*  subjectAltName: host "portcheck.transmissionbt.com" matched cert's "portcheck.transmissionbt.com"
*  issuer: C=US; O=Let's Encrypt; CN=Let's Encrypt Authority X3
*  SSL certificate verify ok.
> GET /51413 HTTP/1.1
> Host: portcheck.transmissionbt.com
> User-Agent: curl/7.58.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< Server: nginx/1.10.3 (Ubuntu)
< Date: Sat, 03 Nov 2018 17:48:54 GMT
< Content-Type: text/html; charset=ISO-8859-1
< Transfer-Encoding: chunked
< Connection: keep-alive
< 
* Connection #0 to host portcheck.transmissionbt.com left intact
1