It definitely could be an out-of-round stem or the handlebar itself. I'm putting my money on the handlebar being out of spec though. There were no unusual marks on the clamp area of the stock aluminum bars. If the stem was out of round it would have at least marred the surface of the handlebars. Pretty sure it was just an out of spec handlebar due to poor QC from cheap Chinese labor.

Out of spec or not, the bars took no real force to crack. They are junk. Even if you are successfully using them right now I'd swap them out for something else asap. The chances of them being durable long-term are not good. Do you really think a $65 handlebar that weighs 175 grams was well engineered? Highly unlikely.