I have seen that caused by excess gas dissolving the unburned oil sludge and carbon left in the cylinders from over mixing the oil in the gas.
try keeping better track of the oil/gas ratio you are using but the simple solution is get plugs one setting hotter and it should burn that gunk off before you see that.
if the mix is right the plugs should be coffee with cream colored light brown.
if this is just a sudden new thing that wasn't there before then maybe you have built in tanks and your oil isn't mixing well and its pulling slugs of unmixed oil from the tank.
also, I know people like to add the gas first so they know how much oil to add but always add the oil before the gas so the gas stirs and mixes the oil well. just don't feel the need to fully top off the tank so you can add x gallons of gas to the oil you added and if its not totally full stop there. rarely do you need a full tank to that extent of accuracy so if your a few gallons shy of full is not a big deal
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