Piston tagged plug??

sometimes the debris from deto or a lean burn in the early phases will hang around the top of the piston and close that gap...hopefully its that and not the crank..

The only time I have seen that happen was on a cat triple drag motor
that they wanted reported for more HP. In taking it apart and cc'ing the heads/combustion chambers found that it had near 17 to 1 compression.
By the end of a 600 ft pass spark gaps were smashed shut. Never blew up
because it only ran for 600 ft. Moeral of the story I lowered the compression a bit and it didn't smash the plugs shut but IT WAS SLOWER.
Sometimes you can't win.

My guess is that engine has to much compression for the octane of fuel
available and its detonating. Mix up some half and half premium and race fuel (100 octane or better) and see if it improves. I saw over the years a lot of engine porters who had no clue about compression ratios and would just cut and hack until they hit the lbs they were looking for on the compression gauge.

An example I was just reading about on DynoTech Research.com was a new cat 800 engine, D&D head with lowered compression, pipe and can, computer tuning and no porting, 170 HP.

Im going to say debris simply because taking into consideration that the plug is at the top of the combustion chamber, the piston couldnt ever touch it unless it was in pieces first.