Did the OP find the root cause of his problem? I'm betting on weak/ permanently compressed springs causing bounce and hammering on the rocker as the cause. Here is a pic of
View attachment 1716150817 1 new, 1 compressed spring. Both from the same batch/ manufacturer built date. The weak/compressed spring caused valve float/bounce leading to hammering on the rocker arm which lead to it's breaking. Excessive/extreme valve lash can have the same results. Backing off rocker arms between long periods of non-use helps longevity. Same for rod bolts in aluminum rods, most quality aluminum rod manufacturers highly recommend it during long periods of non-use.
