I'd be worried. A cracked rocker would simply mean that the related valve would stay shut, and you'd lose that cylinder, not the whole engine.

If the rocker had broken and the broken off part has circulated in the engine and jammed up somewhere, who knows what has happened.

I'm not a mechanic, just an amateur, but if I had changed your chain, and then a rocker broke, I wouldn't consider this a warranty job.

My guess is that they have screwed up, and are going to fix whatever it is they aren't telling you. I just hope they get it right this time!