I gave up a long time ago on checking/monitoring internal temps of pork shoulders because there are just too many variables (size, age of the pig, the breed of hog, what it was fed, etc.). And this is also why I also learned to allow more time than I think I need just in case you get a stubborn shoulder.
I use the "feel" method with a large thermometer to probe them, not for temperature, but for tenderness. When you stick a probe into the shoulder if it slides easily all the way into the middle with little to no resistance, then that means the connective tissues have broken down and the pork is done ready to pull.
If the probe goes part way in then hits resistance, then the it's hitting connective tissues that haven't yet broken down yet and it needs more time. Re-wrap it (it it was wrapped) and leave it on longer until the probe slides all the way in easily.
This method never lies, because it tells you if the connective tissues are broken down or not.