'systemd' kills normal user subprocesses - systemd

Question

I have written a 'systemd's service unit to run a bash script that I have been running, whitout any problems, with '/etc/rc.local' for many years.
The 'systemd's service works perfectly except for the following problem:
My bash script runs, every 5 minutes, the following lines:
nice --adjustment=$CommonNicePriority su root --command="HOME=$COMMON_DIR $0 $CyclesOption" # System common tasks.
for User in $Users # Run logged in users' tasks.
do
nice --adjustment=$UsersNicePriority su $User --command="$0 $CyclesOption"
done
As you can see it spawns a process, running as 'root' (first line) and then one process for every logged in normal user (loop) as (normal) user.
Each of the above processes can spawn some other (task) processes. The processes running as 'root' work all right but those running for normal users are killed by 'systemd'. These are lines from '/var/log/syslog':
Feb 2 10:35:00 Linux-1 systemd[1]: Started User Manager for UID 0.
Feb 2 10:35:00 Linux-1 systemd[1]: Started Session c4 of user manolo.
Feb 2 10:35:00 Linux-1 systemd[1]: session-c4.scope: Killing process 31163 (CommonCron) with signal SIGTERM.
Feb 2 10:35:00 Linux-1 systemd[1]: session-c4.scope: Killing process 31164 (CommonCron) with signal SIGTERM.
Feb 2 10:35:00 Linux-1 systemd[1]: session-c4.scope: Killing process 31165 (CommonCron) with signal SIGTERM.
Feb 2 10:35:00 Linux-1 systemd[1]: Stopping Session c4 of user manolo.
Feb 2 10:35:00 Linux-1 systemd[1]: Stopped Session c4 of user manolo.
Feb 2 10:35:13 Linux-1 systemd[1]: Stopping User Manager for UID 0...
Here is my 'systemd's service:
[Unit]
Description=CommonStartUpShutdown
Requires=local-fs.target
Wants=network.target
[Service]
Type=forking
ExecStart=/etc/after_boot.local
RemainAfterExit=yes
TimeoutSec=infinity
KillMode=none
ExecStop=/etc/before_halt.local
[Install]
WantedBy=local-fs.target
# How I think it works:
# Service starts when target 'local-fs.target' is reached, preferably, when target 'network.target'
# is also reached. This last target is reached even if the router is powered off (tested).
# Service start sequence runs script: 'ExecStart=/etc/after_boot.local' which is expected
# to spawn child processes and exit: 'Type=forking'. Child processes: 'CommonSystemStartUp's
# children: 'CommonDaemon', 'CommonCron'... This script must exit with 0, otherwise systemd
# will kill all child processes and wont run 'ExecStop' script. Service start can run as long
# as it needs: 'TimeoutSec=infinity'.
# Service is kept alive, after running 'ExecStart=...', by 'RemainAfterExit=true'.
# When the service is stopped, at system shutdown, script 'ExecStop=/etc/before_halt.local'
# will run for as long as necessary, 'TimeoutSec=infinity', before target
# 'local-fs.target' is lost: 'Requires=local-fs.target'.
# 'ExecStart=/etc/after_boot.local's children processes ('CommonDaemon', 'CommonCron'...) won't be
# killed when 'ExecStop=/etc/before_halt.local' runs: 'KillMode=none'.
I have tried moving 'KillMode=none' to the [Unit] block: 'systemd' complains.
Also tried 'WantedBy=multi-user.target.wants' in the [Install] block: doesn't make any difference.