How can an existing systemd service become unknown again?

I have a custom system service running on a server, let’s call it xyz.service. It has been running for months without problems and is also enabled to run on reboot (WantedBy=multi-user.target).

When I last checked on it however, it was neither running nor known to systemd:

$ systemctl status xyz
Unit xyz.service could not be found.
$ systemctl list-units | grep xyz
$

The file in /etc/systemd/system and the symlink in /etc/systemd/system/multi-user.target.wants still exist. After a systemctl daemon-reload, the service is available again and (after starting) running fine.

I can see (via journalctl -u xyz) that the service was stopped yesterday.

What could cause this, and is there a way to find out when that happened?