I think I could reproduce the issue.
It seems following entry is missing in /etc/hosts on system A:
127.0.0.1 localhost
After setting the entry rspamd needs to be restarted:
runagent -m mail1 systemctl --user restart rspamd
Now you should get IPv4 and IPv6:
[root@ns8rockytest ~]# ss -tlp | grep rspamd
LISTEN 0 4096 127.0.0.1:11333 0.0.0.0:* users:(("rspamd",pid=639878,fd=73),("rspamd",pid=639877,fd=73),("rspamd",pid=639873,fd=73))
LISTEN 0 4096 127.0.0.1:11332 0.0.0.0:* users:(("rspamd",pid=639875,fd=67),("rspamd",pid=639873,fd=67))
LISTEN 0 4096 127.0.0.1:11334 0.0.0.0:* users:(("rspamd",pid=639876,fd=70),("rspamd",pid=639873,fd=70))
LISTEN 0 4096 [::1]:11332 [::]:* users:(("rspamd",pid=639875,fd=68),("rspamd",pid=639873,fd=68))
LISTEN 0 4096 [::1]:11333 [::]:* users:(("rspamd",pid=639878,fd=75),("rspamd",pid=639877,fd=75),("rspamd",pid=639873,fd=75))