For the past couple weeks I have been experiencing transient failures of fwupd-refresh.service. When I restart the unit I see the following output:

Job for fwupd-refresh.service failed because the control process exited with error code.
See "systemctl status fwupd-refresh.service" and "journalctl -xeu fwupd-refresh.service" for details.

From the aforementioned journal command:

...
... systemd[1]: fwupd-refresh.service: Main process exited, code=exited, status>
░░ Subject: Unit process exited
░░ Defined-By: systemd
░░ Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
░░ 
░░ An ExecStart= process belonging to unit fwupd-refresh.service has exited.
░░ 
░░ The process' exit code is 'exited' and its exit status is 1.
...

There is only one such process in the unit's file, specifying the command /usr/bin/fwupdmgr refresh. When I run this command myself, however, it always succeeds and exits 0. Restarting the unit after running the command manually causes it to no-op (up to date) and succeed.

Ideas to triage these intermittent failures would be quite welcome; I'm at a loss! Why does the service fail when manual invocation invariably succeeds?