Spark Plug query

while investigation my Saito FG30 (four stroke) , i noticed dirt on the spark plug ceramic insulator which appears to be combustion leakage thru the plug. However the rubber seal in the cap has prevented this contaminating the HT contact.

Q Ive never noticed spark plug leakage before on a 2 stroke so wondered if this is normal ?

I think that I usually saw this on engines that have been overheated, thrashed or a lean burn at some time or other. High heat can create minute cracks in the ceramic insulator. This was with cars though but probably quite relevant