Right, I've been out and done some testing today.
I think it is important to make clear that this failure can happen in caving use and would be serious. I think it is also important to make clear that as a personal and professional caver (CIC) that I do not suggest anyone goes and uses a technique that they have doubts about or are unfamiliar with. This includes the use of new fangled French knot thingies which I have yet to test and have only seen formed when people tie a BotB incorrectly.
I will stop short of giving specific advice on this knot but will say that I will continue to use it...