Aftermarket ARBs need to be solid ones to stay on the safer side.
I know stock ARBs also are hollow, but obviously they reach the necessary built quality.
I never heard of broken stock Volkswagen bars, but this isn't the only broken tubular
aftermarket bar. There are others from Hotchkis and APR on several forums showing the
same issue.
If the front sway fails it would even be more serious as the car will oversteer instantly.