I would like to see the underside of the piston, and how the bearing shells clip ( or fall !!) out of the conrod big end. This usually gives a clue. if the bearing shells fall out... indicates heavy detonation. Pinging and detonation create huge loads on the piston and consequently squashing the bearing out of shape. Under the piston, if there is a lot of brown coloration, this can indicate detonation heating the piston and cooking the oil underneath the crown... or just plain simple excessively high Exhaust Gas Temperatures(EGT) overheating the exhaust valve... leading to fatigue failure. The shape of the hole looks pretty irregular, like a chunk has just cracked out. It will be interesting to see if the other valves have distorted faces ?
Was this engine running any form of anti-lag or "pops and bangs" tune ? I've seen similar in Rally motors. The washing of the piston... almost looks a bit normal. I've seen similar on a lot of motors...
