I still get 24+ hour burns but my temps are well below the active zone now after about 16-18 hours. I use to still be above that active zone in that time frame earlier in the season. Maybe the thermometer is off or the cat has just settled in? Still no smoke out the chimney so I'm guessing everything is still working fine?
I'd lean towards the thermometer. I don't think they are exactly precision instruments. Mine acts the same. I compared it to the new one I got for my sister, and hers reads a lot hotter.
Do you ever pull it out and clean it?