You could just have a bad batch. I'd check over on the meat bird forum.
https://www.backyardchickens.com/f/21/meat-birds-etc
I don't think it's shipping stress, or an illness or your other chicks would also have the same problems.
Are these 8 days old or 8 weeks? Heart and leg issues are common with Cornish X. If they are 8 weeks old then they are ready for butchering, and you should do it soon before you lose all of them.