Reactive Kafka - how to correctly perform manual commit?

Question

I am trying to do manual offset commit in following way:

val receiver: KafkaReceiver
val sender: KafkaSender

val flux = receiver.receive() // from one message, I am creating Flux of items which are transformed and then send to another topic 
   .flatMap { Flux.fromIterable() } 
   .map { transform() } 

sender.send(flux) // sending Flux to some other topic
   .doOnNext { logging() }
   .collectList()
   .doOnSuccess {
       val item = it.first()
       item.receiverOffset.commit().subscribe() // I am storing reactors receiverOffset during transformation
    }
   .subscribe()

I would like to commit offset once all items are send to that other topic. Unfortunately, doOnSuccess {} logic is never invoked (surprisingly, logging in doOnNext {} works as expected).

Could someone point me what I am doing wrong? I am pretty new to project reactor (and reactive programming).