Lightdm automatically logs in my user on Ubuntu Xenial when I log out from XFCE

Lightdm automatically logs in my user on Ubuntu Xenial when I log out from XFCE on Ubuntu Xenial.

I have no /etc/lightdm/lightdm.conf file and don't belong to the nopasswdlogin group either :

$ ls /etc/lightdm/lightdm.conf
ls: cannot access '/etc/lightdm/lightdm.conf': No such file or directory
$ groups | grep nopasswdlogin
$

EDIT0: My default DM :

$ cat /etc/X11/default-display-manager
/usr/sbin/lightdm

EDIT1 : I have /etc/lightdm/lightdm.conf.d/10-xubuntu.conf :

$ cat /etc/lightdm/lightdm.conf.d/10-xubuntu.conf
[SeatDefaults]
user-session=xubuntu

EDIT2: Here's what I have in the lightdm.service journal :

$ journalctl -au lightdm -S "2020-01-29 1:59"
-- Logs begin at lun. 2020-01-27 11:14:43 CET, end at mer. 2020-01-29 02:01:47 CET. --
janv. 29 01:59:45 sebastien-B206 lightdm[11808]: Failed to write utmpx: Permission denied
janv. 29 01:59:45 sebastien-B206 lightdm[11808]: Error writing to daemon: Bad file descriptor
janv. 29 01:59:45 sebastien-B206 lightdm[11808]: Error writing to daemon: Bad file descriptor
janv. 29 01:59:45 sebastien-B206 lightdm[11808]: Error writing to daemon: Bad file descriptor
janv. 29 01:59:45 sebastien-B206 lightdm[11808]: Failed to write utmpx: Permission denied

EDIT3: So I've just restarted the lightdm.service :

$ sudo service lightdm restart
$ service lightdm status
● lightdm.service - Light Display Manager
   Loaded: loaded (/lib/systemd/system/lightdm.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2020-01-29 02:09:37 CET; 13s ago
     Docs: man:lightdm(1)
  Process: 15476 ExecStartPre=/bin/sh -c [ "$(basename $(cat /etc/X11/default-display-manager 2>/dev/null))" = "lightdm" ] (code=exited, status=0/SUCCESS)
 Main PID: 15480 (lightdm)
   CGroup: /system.slice/lightdm.service
           ├─15480 /usr/sbin/lightdm
           ├─15490 /usr/lib/xorg/Xorg -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
           └─15618 lightdm --session-child 12 19

Jan 29 02:09:37 sebastien-B206 systemd[1]: Starting Light Display Manager...
Jan 29 02:09:37 sebastien-B206 systemd[1]: Started Light Display Manager.
Jan 29 02:09:38 sebastien-B206 lightdm[15522]: pam_kwallet(lightdm-greeter:setcred): (null): pam_sm_setcred
Jan 29 02:09:38 sebastien-B206 lightdm[15522]: pam_kwallet5(lightdm-greeter:setcred): (null): pam_sm_setcred
Jan 29 02:09:38 sebastien-B206 lightdm[15522]: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Jan 29 02:09:42 sebastien-B206 lightdm[15618]: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "sebastien"

Can you help me disable autologin ?