I don't know if it's particularly the bolts, or if the splines are somehow getting rounded from being torqued one way at the top of the stroke, and the opposite at the bottom, and it eventually worms (not a typo) its way off. With the first two cranks, after the first fall-off I would put it back on and tighten it back up, and it would keep working loose, at shorter intervals. First time for this crank and it would be great if I could halt the pattern here.
Loctite is a good place to start though. I have some loctite blue and will put that on tonight. Maybe I need stronger than blue though.