I've with an old plug that I removed and installed a few too many times had it leak even though torqued down. The metal gasket on some plugs can be removed and if you have a gasket from another less compressed metal gasket it can be swapped in. For the cost of a spark plug I guess you may not bother this way if you have a new one and this turns out to be the reason and can tell it is so.
More recently no mater how you try with some plugs to remove the metal gasket it cannot un-thread sort of. I suppose Dremel will work by cutting a broken plug to save a metal gasket, but how to install?
Only place I hear of spark plug gaskets has to do with some fanatics about spark plug gasket shims to exactly set the depth it reaches in the cylinder.
What I noticed was not bubbling, but I have a four stroke engine, it was a carbon deposit near one side of the spark plug. After cleaning the spot and newer plug with better metal gasket it never showed carbon around the outside of the cylinder head near the spark plug again.