I think the difference between kneeling and sitting is that when your kneeling it's easy to fall over, or for your body position to change once unconscious. If your in a more seated posture, there's less chance of drastic changes in body positioning imo, so less chance for the knot to loosen, imo.
There are numerous reports not unlike your own online, they are disturbing but I have never seen footage of said scenario so it's hard to really know what exactly happened.
I have tied a few different knots and one in particular (8 chan, picture diagram for the slipknot) with a nylon rope was extremely hard to loosen, I think having a rope that will hold tightly after your out is extremely important. My static climbing rope doesn't have the same hold and loosens easily, so i'd probably only use that one for full.