Just to satisfy my curiosity, and since the lash caps look just like what is in the photo on your intake valve, would you check to see if that is a cap on there, or not?
It should just pull right off with a pair of needle-nose pliers, without any damage.
If there are no lash caps anywhere to be found, then it was assembled without lash caps, and that's what caused it.
If there is a lash cap on the intake, and not on the exhaust, then that could be the reason why the exhaust failed. Or perhaps the exhaust lash cap broke apart. I have seen them severely damaged on the top before, but never actually coming apart. But it could happen.