Eu consertei isso criando o arquivo /etc/tmpfiles.d/tmp.conf contendo:
# Override cleaning of /tmp in vendor /usr/lib/tmpfiles.d/tmp.conf,
# as documented in tmpfiles.d(5), for Kubuntu 15.04 and above that
# use systemd. First char is 'd', instead of 'D', to avoid cleaning.
d /tmp 1777 root root -