cat roller seized engine valve

im puzzled got a major problem,i just overhauled a cat cs 533 e engine 3054(perkins by the way)
put in new pistons,bearing etc,after some good hours of running in went for a test full throttle with vibration on and it starts missing about 20 mins later.
finally opened the valve cover and found a push rod had popped out because an inlet valve number one had siezed.

putting in a new valve and all the valve guides although they are fine.
anyone had such an issue coz i cant figure out why this...

cat roller seized engine valve

cat roller seized engine valve