I have 2 plug wires that touch the headers even with these short plugs. I have those two wires wrapped with protective heat shields so they don't melt the boots.
That might be your problem. When you leave and the header heats up, the boot could be allowing the spark to jump. The more timing you add the hotter the cylinder is and could accelerate the problem also. I ran the sleeve over the wire on one engine and it caused the wire to burn badly for several inches. Removed wire, cut off damaged portion, and put it back on WITHOUT the sleeve and had no more problems. Just trying to help a neighbor as I'm located in Ocala.