The bone retains heat and I bet you are overcooking the meat next to the bone. You are smoking the butt more like a brisket in my opinion. The pork has a higher fat content and can be cooked at a higher temp and less time. I smoke mine at 275 and spritz it every hour for about three hours until there is a nice bark and the fat cap splits. Then I wrap it with foil and start temping after two hours until I hit somewhere in the 200 -205 deg range, make sure not to temp the bone inside. Pull it and let rest for an hour.