This was a weird one. It took me ages to discover why SSH X11 died at some point after tuning a box.
It so happens that disabling IPv6 ( not used on my networks) is assumed in SSH at some point and if you disable it (which many people say is a way to avoid systems using IPv6 to other boxes and ignore IPv4 provisions like firewall settings, routes etc..)
So re-enable the IPv6 in /etc/sysctl.conf or /etc/sysconfig/network or other networking script locations, if you need this functionality.
If i find any other counter indication I’ll drop a note and link.