I had the same problem with my '84, I caught it before it snapped, but the shaft was bent at the bottom similar to where yours broke.
I believe the problem is that the linkage mounting position on the bottom of the shock does not sit directly underneath the shock shaft, it is offset by about 20mm. The maico shock is the only one I have seen mounted in his fashion. As pressure is applied to the shock from the linkage, the load is not transfered directly to the shock shaft, but slightly behind it, putting stress on the joint between the shaft and the bottom mount, just where yours broke.