When I read the title of your post, my first thought was that you are a 3-under shooter. The vertical bow doesn't help either, but it is not the problem by itself.
However, it is also my style of shooting. I find that it goes well with split vision (I do shoot instinctive on shots requiring cants and other precarious drawing positions).
My arrow also comes off the riser when I draw. Especially with aerial shooting and taking snapshots. It is because as 3-under shooters, we have less control over the back of the arrow, since a split-finger-hold shooter literally has his fingers wrapped around the back of the arrow, he has more control and is able to use a slight rotational pressure with his fingers to hold the arrow against the riser.
The solution? I just lift my index knuckle as I draw to give support to the arrow and tuck it back down when I reach anchor. It is something I never even think about. They say this is for kids, but it's not. If your form is good and you need to lift a knuckle to draw, big deal. It will enable you to shoot fast, and this is a complete advantage in hunting situations, especially from the ground.
The trick from the John shulz video about rolling the string might help you. However, it only worked for me when I was a split-finger-hold shooter. It seems like the lack of finger control on the arrow is exactly the problem.
What Terry Green mentioned about reversing cant is completely correct. However, I can only take this shot with a split draw. When shooting 3-under, I cannot impart enough pressure by rolling the string to hold the arrow against the riser during a reverse cant. I have had to tolerate the loss of this shooting position since my switch, although I'd love to get it back.