The specified systemd .service file contains both DefaultDependencies=no and Conflicts=shutdown.target directives without Before=shutdown.target.
This can lead to problems during shutdown because the service may linger until the very end of shutdown sequence as nothing requests to stop it before (due to DefaultDependencies=no).
There is race condition between stopping units and systemd getting a request to exit the main loop, so it may proceed with shutdown before all pending stop jobs have been processed.
Please add Before=shutdown.target.
Refer to https://github.com/systemd/systemd/issues/11821 for details.