Seems like there should have been some detonation/pinging if it was lean enough to seize the piston in the cylinder. If the damage is on the exhaust side of the cylinder and piston only, you were likely running lean.
Any sign of detonation on the piston crown?
Any aluminum on the plug? Look closely with a magnifying glass.
Check the bottom of the piston for a heat pattern.
Was the bike warmed up properly? Sufficient lubrication? If you have a four corner seizure it may be due to cold seizure or insufficient lubrication.
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