Consider FLUSH.startFlush():

If the timeout expires on the flush_promise, the backoff-sleep algorithm kicks in. This sleeps for a random time and then tries calls startFlush again. The problem is that between the time that the flush_promise times out and startFlush is re-issued, the flush could have completed successfully, but it would go unnoticed, because after Util.sleep completes, the flush_promise is reset.

Recommend using the flush_promise.getResultWithTimeout(timeout) alone and not Util.sleep(). The group is just as operational during flush_promise.getResultWithTimeout(timeout) as in Util.sleep().