Without a sophisticated ipv6 setup I tried the most trivial.
# ssh ip6-localhost
The authenticity of host 'ip6-localhost (::1)' can't be established.
[...]
root@zesty-test:~# ssh -4 ip6-localhost
The authenticity of host 'ip6-localhost (127.0.0.1)' can't be established.
As you can see the -4 successfully avoided to use the ipv6 address.
What version/release are you running on, any details on your setup that might help to understand?
If you could use apport-collect that would add not all (not your setup details), but most other info needed.
Without a sophisticated ipv6 setup I tried the most trivial.
# ssh ip6-localhost
The authenticity of host 'ip6-localhost (::1)' can't be established.
[...]
root@zesty-test:~# ssh -4 ip6-localhost
The authenticity of host 'ip6-localhost (127.0.0.1)' can't be established.
As you can see the -4 successfully avoided to use the ipv6 address.
What version/release are you running on, any details on your setup that might help to understand?
If you could use apport-collect that would add not all (not your setup details), but most other info needed.