How do I stop a unit that consists of multiple processes? - systemd

Question

In Ubuntu 22.04
I have two service units that I have written.
One is terminated without problem by the ExecStop=/bin/kill -1 $MAINPID in the service file.
However, the other does not terminate normally in the same way as above.
$ sudo systemctl stop foo
$ systemctl status foo
× foo.service - foo server
Loaded: loaded (/etc/systemd/system/foo.service; disabled; vendor preset: enabled)
Active: failed (Result: timeout) since Thu 2023-02-16 19:28:31 KST; 2min 24s ago
Process: 141572 ExecStart=/opt/foo/sbin/foo_daemon/foo.py (code=killed, signal=HUP)
Process: 142097 ExecStop=/bin/kill -1 $MAINPID (code=exited, status=0/SUCCESS)
Main PID: 141572 (code=killed, signal=HUP)
CPU: 685ms
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Failed with result 'timeout'.
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Unit process 141605 (foo.py) remains running after unit stopped.
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Unit process 141606 (foo.py) remains running after unit stopped.
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Unit process 141607 (foo.py) remains running after unit stopped.
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Unit process 141608 (foo.py) remains running after unit stopped.
Feb 16 19:28:31 esm-op systemd[1]: foo.service: Unit process 141609 (foo.py) remains running after unit stopped.
Below is my foo.service file.
[Unit]
Description=foo server
StartLimitIntervalSec=60
StartLimitBurst=3
[Service]
Environment="LD_LIBRARY_PATH=/opt/foo/lib"
ExecStart=/opt/foo/sbin/foo_daemon/foo.py
RestartSec=10
SuccessExitStatus=1 143 11
ExecStop=/bin/kill -1 $MAINPID
[Install]
WantedBy=graphical.target
Can someone help me?