I had a bolt fall out of a Great Lake Serria xbow when held in a vertical position from my treestand last season, may have been a weak spring, don't know,as I had borrowed it. I checked this on my vixen, it held the bolt tight.
Just a word of caution. One night when I was taking off my string from the limbs, I was using my "destringer", not sure if that's the correct wording. Its the black stringer with the two square plastic holders that go around the mag limb tips. Anyway, I wasn't paying attention and accidentally brought the destringer over top of my bolt spring holder. I bent the bolt holder, but it seemed to bounce back into shape. Not sure if the spring can be permanently damaged, but don't want to find out.