After each logout from KDE, I can find the systemd --user process when I log into a tty{2..6} console and its remaining 23 children still running :
$ ps -Af | grep -v grep | egrep -w "PPID|$(pgrep -u $USER systemd)"
UID PID PPID C STIME TTY TIME CMD
sebasti+ 64862 1 0 13:13 ? 00:00:00 /lib/systemd/systemd --user
sebasti+ 64865 64862 0 13:13 ? 00:00:00 (sd-pam)
sebasti+ 64881 64862 0 13:13 ? 00:00:00 /usr/libexec/tracker-miner-fs
sebasti+ 64884 64862 0 13:13 ? 00:00:01 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
sebasti+ 64901 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfsd
sebasti+ 64906 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfsd-fuse /run/user/1001/gvfs -f -o big_writes
sebasti+ 64908 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfs-udisks2-volume-monitor
sebasti+ 64956 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfs-gphoto2-volume-monitor
sebasti+ 64971 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfs-goa-volume-monitor
sebasti+ 64975 64862 0 13:13 ? 00:00:00 /usr/libexec/goa-daemon
sebasti+ 64998 64862 0 13:13 ? 00:00:00 /usr/libexec/goa-identity-service
sebasti+ 65004 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfs-afc-volume-monitor
sebasti+ 65010 64862 0 13:13 ? 00:00:00 /usr/libexec/gvfs-mtp-volume-monitor
sebasti+ 65020 64862 0 13:13 ? 00:00:00 /usr/lib/x86_64-linux-gnu/tumbler-1/tumblerd
sebasti+ 65339 64862 0 13:13 ? 00:00:01 /usr/libexec/gvfsd-metadata
sebasti+ 65855 64862 0 13:14 ? 00:00:00 /usr/libexec/dconf-service
sebasti+ 66233 64862 0 13:14 ? 00:00:00 /usr/bin/zeitgeist-daemon
sebasti+ 66247 64862 0 13:14 ? 00:00:00 /usr/lib/zeitgeist/zeitgeist-fts
sebasti+ 66341 64862 0 13:14 ? 00:00:00 /usr/libexec/xdg-document-portal
sebasti+ 66356 64862 0 13:14 ? 00:00:00 /usr/libexec/xdg-permission-store
sebasti+ 66463 64862 0 13:14 ? 00:00:00 /usr/lib/telepathy/mission-control-5
sebasti+ 66624 64862 0 13:14 ? 00:00:00 /usr/lib/bluetooth/obexd
sebasti+ 66793 64862 0 13:14 ? 00:00:00 /usr/libexec/xdg-desktop-portal
sebasti+ 69972 64862 0 13:20 ? 00:00:00 /usr/bin/pulseaudio --daemonize=no --log-target=journal
$ loginctl list-sessions
SESSION UID USER SEAT TTY
99 1001 sebastien seat0
79 126 sddm seat0
105 1001 sebastien seat0 tty2
3 sessions listed.
$ loginctl session-status $(loginctl list-sessions | awk -v user=$USER '$0 ~ user".* $"{print$1;exit}')
99 - sebastien (1001)
Since: Sat 2021-08-07 15:52:39 CEST; 13min ago
Leader: 119723
Seat: seat0; vc1
Display: :0
Service: sddm; type x11; class user
Desktop: KDE
State: closing
Unit: session-99.scope
├─120287 /usr/bin/baloo_file
├─120342 /usr/libexec/at-spi-bus-launcher --launch-immediately
├─120349 /usr/bin/dbus-daemon --config-file=/usr/share/defaults/at-spi2/accessibility.conf --nofork --print-address 3
├─120407 /usr/libexec/geoclue-2.0/demos/agent
├─120439 /usr/bin/zeitgeist-datahub
└─122590 /usr/libexec/at-spi2-registryd --use-gnome-session
Aug 07 15:52:39 michel-K72Jr systemd[1]: Started Session 99 of user sebastien.
Aug 07 15:52:40 michel-K72Jr sddm-helper[119790]: pam_kwallet5: final socket path: /run/user/1001/kwallet5.socket
Aug 07 15:52:40 michel-K72Jr sddm-helper[119795]: Adding cookie to "/home/sebastien/.Xauthority"
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: Checking for pam module
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: Got pam-login param
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: Waiting for hash on 7-
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: waitingForEnvironment on: 3
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: client connected
Aug 07 15:53:09 michel-K72Jr sddm[119792]: kwalletd5: client disconnected
Aug 07 15:53:09 michel-K72Jr sddm-helper[119723]: [PAM] Closing session
Aug 07 15:53:09 michel-K72Jr sddm-helper[119723]: pam_unix(sddm:session): session closed for user sebastien
Aug 07 15:53:09 michel-K72Jr sddm-helper[119723]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_close_session
Aug 07 15:53:09 michel-K72Jr sddm-helper[119723]: pam_kwallet5(sddm:setcred): pam_kwallet5: pam_sm_setcred
Aug 07 15:53:09 michel-K72Jr sddm-helper[119723]: [PAM] Ended.
Maybe this is related to sddm and its child processes :
$ loginctl show-session $(loginctl list-sessions | awk -v user=$USER '$0 ~ user".* $"{print$1;exit}')
Id=99
User=1001
Name=sebastien
Timestamp=Sat 2021-08-07 15:52:39 CEST
TimestampMonotonic=19979613418
VTNr=1
Seat=seat0
Display=:0
Remote=no
Service=sddm
Desktop=KDE
Scope=session-99.scope
Leader=119723
Audit=99
Type=x11
Class=user
Active=no
State=closing
IdleHint=no
IdleSinceHint=0
IdleSinceHintMonotonic=0
LockedHint=no
$ ps -Af | grep -v grep | egrep -w "PPID|$(pgrep sddm)"
UID PID PPID C STIME TTY TIME CMD
root 2240 1 0 10:19 ? 00:00:00 /usr/bin/sddm
root 120872 2240 10 15:53 tty1 00:03:49 /usr/lib/xorg/Xorg -nolisten tcp -auth /var/run/sddm/{3ef665c9-84b6-413b-87e1-918808539125} -background none -noreset -displayfd 18 -seat seat0 vt1
root 121865 2240 0 15:56 ? 00:00:00 /usr/lib/x86_64-linux-gnu/sddm/sddm-helper --socket /tmp/sddm-auth6612f95d-2348-42b5-b4ae-8247c8f4d0af --id 19 --start /usr/bin/startplasma-x11 --user sebastien
sebasti+ 121890 121865 0 15:56 ? 00:00:00 /usr/bin/startplasma-x11
$ ps -Af | grep -v grep | egrep -w "PPID|$(pgrep startplasma-x11)"
UID PID PPID C STIME TTY TIME CMD
sebasti+ 121890 121865 0 15:56 ? 00:00:00 /usr/bin/startplasma-x11
sebasti+ 122249 121890 0 15:56 ? 00:00:00 /usr/bin/ssh-agent /usr/bin/im-launch env LD_PRELOAD=libgtk3-nocsd.so.0 /usr/bin/startplasma-x11
sebasti+ 122298 121890 0 15:56 ? 00:00:00 /usr/bin/plasma_session
Created a bug report at : https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1939196
