I've never seen a spade and only relying on what I've read elsewhere. From what I understand the bolt on a spade merely holds it together when stowed or until it gets to the bottom. Once on the bottom, the natural pressure of the anchor
design forces a mechanical bond of the two pieces. The bolt is under no force in use.
I suppose it is possible for the bolt to fall out and allow the two parts
of the anchor
to separate before it hits the bottom. but you'd have to be awfully unlucky. In the anchor that 'failed', there is the possibility that the bolt either was never installed or installed improperly allowing the two halves to separate when they were launched.
Must interject my caveat one more time, I've never seen a spade up close and personal just going by others reports.