I have a feeling it is not the spacer company's fault, more likely it is improper torquing of the lug nuts, or The torque got changed over time, that put uneven pressure across the surface.
No amount of suspension moving up and down is going to impact an aluminum spacer. The spacer is moving axially (microscopically) in place while it's rotating radially due to improper lug nut torque could be a potential cause of this crack.
Even if a torque wrench was used religiously during the life of the spacer, a minute amount of dirt trapped between the spacer and the surface of the hub, or between the spacer and the rim, would have introduced a point load so severe that it would've cracked a 1.5 in thick aluminum.
If it was a manufacturing defect, it would have shown its ugly head long before 200,000 miles.
Think like a scientist, since you are one