Bad Flatwound string.
Hard to say exactly but some LaBellas use a thinner core (especially on their Vintage style strings) which may have snapped at the saddle or somewhere along the length of the string. On a Thru-body, the "Break Angle" over the saddle is more acute, and can stress the core beyond its limit depending on the string's construction. Flatwounds especially are stiffer, and the Ribbon Wrap may cut into the core with this type of string load. You can retro-fit the bass with a top-load bridge from Allparts, Mighty Mite, Leo Quan or try a different series of LaBella Flats. They make several and they're all good.
Ian "Conjureman" Aniano