The joint on my cerium belt broke, is it defective?

Most likely no.

The cerium belts are a film backed belt and thus can lose tracking if you have an older sander where the crown on your drive wheels has worn down. 
If this is the case, many people will instead just increase the tension on the belt to hold tracking on it. This puts undue stress on the joint and causes prematurely failure.

If your cerium belt is not tracking well, you need to check your drive wheels to see if they still have a crown on them. If not, you'll need to either re-machine the drive wheels, replace them with new wheels, or build a crown on the wheel with masking tape in order for the cerium belts to track correctly without increasing tension.