I have now found the documentation for ExecStop (in systemd.service(5)), which hopefully improves my understanding.

It definitely seems to be both "cause to stop" and "after (unexpected) stop" in one. You can look at $MAINPID to see which case you have. This design apparently makes sense to you, but to me and several others in this thread a service that has already stopped isn't in need of being stopped and shouldn't execute commands intended for that. (There is a separate ExecStopPost for "after stopping, for any reason".)