We now use Flux.flatMapSequential(…) instead of concatMap as concatMap reduces the request size to 1. The change in backpressure/request size reduces parallelism and impacts the batch size by fetching 2 documents instead of considering the actual backpressure.
flatMapSequential doesn't tamper the requested amount while retaining the sequence order.
Closes: #4543
Original Pull Request: #4550
fcc222244b