I get reasonable output from x2golistsessions_root. I also see a matching directory in /tmp/.x2go-$user/ containing 3 files:
[root@hercules C-andrew-51-1513021653_stDMATE_dp24]# ls total 12 -rw-rw-r-- 1 andrew andrew 1 Jan 10 08:56 options -rw-rw-r-- 1 andrew andrew 12 Jan 10 08:56 sshd.pid -rw-rw-r-- 1 andrew andrew 8 Jan 10 08:56 state [root@hercules C-andrew-51-1513021653_stDMATE_dp24]#
Hm, that doesn't look right. A suspended session's directory should contain many more files, like a spool symlink, a cache directory, a mimebox symlink, an options file, a cmdoutput file, a state file and maybe others.
I really do believe that something is deleting files in /tmp/ without you knowing it!
Did you really check for a cron job or the like that might clean up /tmp/? It looks like CentOS 7 has this configurable in /usr/lib/tmpfiles.d/tmp.conf and uses systemd for that purpose.
Mihai