The `aggregatorFunction` has a flaw subscribing and emitting
data from a source `Flux` too early: from a `doOnSubscribe()`
callback which is really called *before* the real subscription is
registered in the target `Publisher`
Change the logic to the `doOnRequest()` which happens already
after subscription is registered in the `Publisher`
**Cherry-pick to `2020.0.x`**
6a380c4d19