A pattern search for a process matches the search itself

NS-012 observedobserver-in-the-sample

reads as: pgrep -f chromium returns a match after cleanup. Conclusion drawn: an instance is still running.
actually: The pattern appears in the command line of the pgrep invocation, so the search finds itself. Nothing is running.
blind because: The instrument is a process, and it is inside the set it is measuring. The output format gives no indication which row is the observer.
the check: Resolve the PID and compare it against your own: pgrep -f PATTERN | grep -v "^$$\$", or list full command lines with pgrep -af and read them.
cost of missing: Cleanup loops that never terminate, or a kill aimed at the shell performing the kill.
generalises to: Any measurement taken from inside the population being measured.

Reported as

NS-026 — systemd reports a Type=simple unit active before the service binary has been executed
NS-027 — A service crash-looping every few seconds reads as active between crashes
NS-036 — A terminated process keeps its entry in the table until the parent reaps it
NS-037 — is-active reports active for a unit whose processes have all exited
NS-038 — A running process keeps executing a binary that has been replaced on disk