I suppose systemd is trying to kill the wrong PID. Eventually, the kill operation times out and systemd hard-kills the process. This happens, for example, when trying to reboot or halt the computer.

Arch Linux, everything up to date.
Nodm version 0.12

Here's the systemd service file:

[Unit]
Description=nodm display manager

[Service]
EnvironmentFile=/etc/nodm.conf
ExecStart=/usr/bin/nodm

[Install]
WantedBy=multi-user.target