systemd --user children processes not stopped afer logout

Hi,

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 -n 15 session-status $(loginctl list-sessions | grep -v tty | grep -m1 $USER | awk '{print $1}')
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

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: xorg 1:7.7+19ubuntu14
ProcVersionSignature: Ubuntu 5.4.0-80.90-generic 5.4.124
Uname: Linux 5.4.0-80-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
BootLog:

CasperMD5CheckResult: skip
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: None
CurrentDesktop: KDE
Date: Sat Aug 7 13:40:23 2021
DistUpgraded: 2020-12-26 17:43:58,639 DEBUG Running PostInstallScript: './xorg_fix_proprietary.py'
DistroCodename: focal
DistroVariant: ubuntu
ExtraDebuggingInterest: I just need to know a workaround
GraphicsCard:
 Advanced Micro Devices, Inc. [AMD/ATI] Park [Mobility Radeon HD 5430/5450/5470] [1002:68e0] (prog-if 00 [VGA controller])
   Subsystem: ASUSTeK Computer Inc. Park [Mobility Radeon HD 5430/5450/5470] [1043:1bf2]
InstallationDate: Installed on 2015-12-27 (2049 days ago)
InstallationMedia: Kubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
MachineType: ASUSTeK Computer Inc. K72Jr
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-5.4.0-80-generic root=/dev/mapper/VG_ALL_Samsung_SSD_850_EVO_500GB-rootFS ro
SourcePackage: xorg
Symptom: display
UpgradeStatus: Upgraded to focal on 2020-12-26 (223 days ago)
dmi.bios.date: 09/06/2010
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: K72Jr.209
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: K72Jr
dmi.board.vendor: ASUSTeK Computer Inc.
dmi.board.version: 1.0
dmi.chassis.asset.tag: ATN12345678901234567
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK Computer Inc.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrK72Jr.209:bd09/06/2010:svnASUSTeKComputerInc.:pnK72Jr:pvr1.0:rvnASUSTeKComputerInc.:rnK72Jr:rvr1.0:cvnASUSTeKComputerInc.:ct10:cvr1.0:
dmi.product.name: K72Jr
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK Computer Inc.
version.compiz: compiz 1:0.9.14.1+20.04.20200211-0ubuntu1
version.libdrm2: libdrm2 2.4.105-3~20.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 21.0.3-0ubuntu0.2~20.04.1
version.libgl1-mesa-glx: libgl1-mesa-glx 21.0.3-0ubuntu0.2~20.04.1
version.xserver-xorg-core: xserver-xorg-core 2:1.20.11-1ubuntu1~20.04.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.6-1
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:19.1.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20200226-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.16-1
xserver.bootTime: Sat Aug 7 13:16:54 2021
xserver.configfile: default
xserver.errors:
 Failed to open authorization file "/var/run/sddm/{e1da4916-44a6-4842-ba41-2c1b0a4fb76b}": No such file or directory
 RADEON(0): failed to set mode: Permission denied
xserver.logfile: /var/log/Xorg.0.log
xserver.outputs: Output HDMI-0 LVDS VGA-0
xserver.version: 2:1.20.11-1ubuntu1~20.04.2
xserver.video_driver: radeon