The washer is a special belleville washer, domed outward so it provides some spring loading on the harmonic balancer. A flat washer will not provide this "springiness", and so the bolt has a greater tendency to back out under the vibrations that the crank and bolt and harmonic balancer experience. The harmonic vibrations are transmitted down the crank to the balancer, which damps them, both thru the crank and the bolt, and this tends to make to bolt "walk" out of the threads. The belleville washer will absorb some the vibrations, lessening the effect on the bolt. I think yours backed our solely because of the wrong washer. I've seen some that I suspect had a bolt failure and some wallowing out of the balancer bore, and then were put back together without replacing the balancer, and the wobbling balancer caused a subsequent bolt breakage from the bending due to balancer wobble.

I would not wire the bolt, I'd just use a small dab of red threadlocker and the right washer.