I don't think that is uncommon. I get that with my boiler. Clinkery type stuff. I try to keep a layer of it on top of my refractory - it's been cracked for going on 6 years now and I haven't touched it. Likely mainly because of that layer of stuff I try to keep there.
I was never able to run it long enough to experience that kind of buildup. I think that is a good sign.