Host Key Verification Strategy is broken for non-standard SSH ports.
[01/19/20 06:17:44] [SSH] Opening SSH connection to slave.net28:58968.
[01/19/20 06:17:44] [SSH] WARNING: No entry currently exists in the Known Hosts file for this host. Connections will be denied until this new host and its associated key is added to the Known Hosts file.
Key exchange was not finished, connection is closed.
java.io.IOException: There was a problem while connecting to slave.net28:58968
jenkins@jenkins:~$ ssh-keygen -H -F [slave.net28]:58968
|1||EAUuHpVvln52WKE434qHFyJrEzM=||KyIhaIA1YlW1hDeFIzdvgJQzU8s= ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBFmmZQx/Cmy1rRV7HsAff4JiIqKPopwVtIgkAaAnG38DESY/cG4xYum0i96eYzmvGxf4UADKDT2e7ePFkJmp9yM=|
Changing ssh port to 22 fixes the issue.