Is the observable setup async?

Question

Lets consider the following example:

// Subject sources have been created from Subjects

const one$ = scheduled([firstSubjectSource$, secondSubjectSource$], asyncScheduler)
  .pipe(
    mergeAll(),
    share(),
  );
const two$ = scheduled([thirdSubjectSource$, fourthSubjectSource$], asyncScheduler)
  .pipe(
    mergeAll(),
    share(),
  );
const final$ = scheduled([one$, two$], asyncScheduler)
  .pipe(
    combineLatestAll(),
    map(() => { /* Some mapping */ }),
  );
return final$;

The final$ is created, returned and can be subscribed to.

I have observed that the marble tests work perfectly, i.e, by the time the tests run all the observables have been setup and subscribed to correctly. But in the actual executing environment (iOS 15 JavascriptCore), this doesn't seem to be the case. Values are forwarded to the ...SubjectSource$ observables after subscription to final$, but final$ never emits anything. Tapping console logs in the one$, two$ shows that they also don't emit anything. My current hypothesis is that the internal subscription process hasn't finished. I have combed through some rxjs code but it doesn't look like the subscription process is async.

AFAIK, the asyncScheduler shouldn't make the internal subscription async. It should only affect how the input values are processed and forwarded.

If the return statement is changed to below, then everything works fine. However, putting an arbitrary wait time doesn't seem like the correct thing to do either.

setTimeout(() => cb(final$), 100);

1. Is the internal setup of observables one$, two$ and final$ async or sync?
2. Is there an event that I 'need to'/'can' wait on before returning final$ for use?

How do I make sure that the observables are actually ready for use before I return it?