I spent a while git-bisecting this and it comes down to this commit:
21b7e38
Before this commit, if you did:
./ncat/ncat --proxy-type http --proxy MYPROXY:8080 ENDPOINT
Then what was sent to MYPROXY was:
CONNECT [2611:1a:e10::4]:22 HTTP/1.0\r\n
Where 2611:1a:e10::4 is the IPv6 address of ENDPOINT.
After this commit, this is sent:
CONNECT ENDPOINT:22 HTTP/1.0\r\n
Which the proxy may not be able to resolve which causes a failed to resolve error, which then breaks being able to proxy.