Agree, definitely replace that coil. It should never arc from the exterior like that, even if the spark plug was "open circuited" as the insulation on the coil body, etc. should never leak current.I would think that there must be a break in the insulation or the epoxy at the top of the pack. I would start by replacing that one and see if it fixes the issue. I can't think of what else would do that.
I posted what the issue was.Agree, definitely replace that coil. It should never arc from the exterior like that, even if the spark plug was "open circuited" as the insulation on the coil body, etc. should never leak current.