I'd be interested in seeing the faceplate of the stem that was clamping these bars.

If it was as described, a cheap arsed alloy stem meant for cheap arsed alloy bars then there is a good chance that the stemp clamp was not 100% round and/or the mating surface was not designed for carbon and concentrated all the clamping force in too small an area.

Sorry to hear about the bars failing regarless of the reason.