Have seen likewise -but not so much- carbon build up after using "over stored" gasoline in combination with too much oil, (as some amount of the gas had evaporated)
After exchanging the fuel mixture, the effect was over.
I'am now also using a colder Plug in my bike when just putting around, for faster riding with modultated terrain and long inbetween tracks I use the standard plug. Works pretty good so far.
To determine the engine health by plug color has got difficult, this is slightly too lean:
That is OK: