Zinc pest is almost always due to high lead content. I'll look for the report I have, but as I recall, all it takes is 50ppm or more lead and you have trouble. Chemical analysis is required to determine lead content of a melt. You cannot tell just by looking at it. The small smelters in China do not have the where-with-all to get this testing done. So until the current brands move their production facilities out of China the problem is likely to continue.
My 2 cents.
Chris
LVHR