Effect of Water/Meth injection on spark plugs

[FlyingGrape]

Currently working on a turbo charged petrol engine. We are running 50/50 mix water/meth and at the moment just under 20 pounds of boost. We have a 4 port water injection system (4 cylinder engine). AFR's are between 12.2-12.6:1 (On a low boost setup these engine seem to work well with AFR's around 12.2:1 range and dont need a lot of combustion cooling).

We have noticed that the spark plug strap, ceramic core and electrode are come out very clean but down in between the thread body and base of the electrode it is quite sooty.

Is this likely that water/meth is cleaning the top of the plug? Any suggestion on why we are seeing this heavy soot inbetween thread and electrode. Is plug perhaps too cold given cooling of combustion from water /meth?