Could be crappy soft cake breaking apart, the kind that forms when people give advice like "shake the ash, then dump it and leave the ash on the walls" kind of crap suggestions.
Could be the beginnings of a burn out.
Either way, you can't tell until you reduce the existing cake to a bare film over the wood, Then you get to see what's what. If it turns out to be heat cracks or a burn through, those can be remedied. Go gently into that cake so that you don't accidentally gouge the wood. Toward the finish I'd put away the commercial reamers and go with glass paper wrapped around something similar in shape to a wide Pilot marking pen case. Then carefully remove more of the cake.
Just don't go looking for informed advice on YouTube. There is some there, but it's like Sturgeon's Law, 90% is crap.
Ok, sorry, I'm being kind, more like 99.9% pure crap.