Comment # 6 on bug 1212932 from Juha Virtanen

For me it seems like /usr/lib/tmpfiles.d/sddm.conf is ignored:

5 systemd-analyze cat-config tmpfiles.d | grep " /tmp/"
x /tmp/systemd-private-%b-*
X /tmp/systemd-private-%b-*/tmp
R! /tmp/systemd-private-*
D! /tmp/.X11-unix 1777 root root 10d
D! /tmp/.ICE-unix 1777 root root 10d
D! /tmp/.XIM-unix 1777 root root 10d
D! /tmp/.font-unix 1777 root root 10d
r! /tmp/.X[0-9]*-lock
$ cat  /usr/lib/tmpfiles.d/sddm.conf 
# Home dir of the sddm user, also contains state.conf
d       /var/lib/sddm   0750    sddm    sddm
# This contains X11 auth files passed to Xorg and the greeter
d       /run/sddm       0711    root    root
# Sockets for IPC
r!      /tmp/sddm-auth*
# xauth files passed to user sessions
r!      /tmp/xauth_*

I think, like Github discussion also seems to do, that sddm.conf should also
have line

X /tmp/xauth_* - - - -