Comment # 3 on bug 919673 from Bernhard Wiedemann

/usr/lib/tmpfiles.d/tmp.conf configures /tmp to not be cleaned
(there was a long discussion on this some years ago
with people losing files from the unexpected auto-cleaner)

but as sysadmin, you can override it via 
/etc/tmpfiles.d/tmp.conf containing
d /tmp 1777 root root 7d