Hmmmm... I'm having a hard time visualizing slight up and down movement causing the bowstring to come off the peg.

Here's a pic of the Truball Short and Sweet in action I snipped off AT (sorry for the size)-




Good close up detail. Appears that the bowstring would fit in the peg quite nicely but I don't know...

Just caught your edit. I could see that possibly being an issue to contend with...