This bug was introduced in #3504, which intended to not subscribe to the source if the notifier synchronously emits a value, but instead it only skipped subscribe if it completed

https://stackblitz.com/edit/js-j72dzx?file=index.js

import { of, empty } from 'rxjs';
import { takeUntil } from 'rxjs/operators';

// does not log anything, but it should
of(1, 2, 3).pipe(
  takeUntil(empty())
).subscribe(d => console.log(d));

Like most bugfixes this could potentially be a breaking change if someone is knowingly or unknowingly relying on this fact. Dunno what the current policy is on breaking bug fixes.