Hah. I have noticed a version of this. If quick reply is at the top of the screen while you post using it, reply is posted but browser doesn't scroll down to show it. It might give the impression that reply just disappeared. Not sure if Andy is talking about this.
Thank you for reporting this issue. This was fixed later, in 2.3.8, through a newer report of the same problem: after posting with quick reply, the page now scrolls to the new post if it isn't already in view.