They just suck into the frame, correct? I don't think they are coming undone. I've strung the 18g before and it's a pain in the butt.
You can try what is called a "Deadman's" knot. This is where you take a piece of scrap string and "seat" it under your knot(s).
If i were you I'd take a small piece of poly or syn gut, and slip it in between the two legs of string right beneath your knot. (Insert it before you cinch up your first knot tie). This will help create a "seat" for your knot, and helps prevent sucking into the frame. However, the grommets will tend to get a little mashed using this technique.