Ravel, I believe you experienced what you did by overloading the #2. First, 4 x 9 lb butts in a #2?? Wow! They had to be jammed in there pretty tight! I think you most-likely restricted the heat flow to the wall sensor. You had 36 lbs of cold meat between the heat source and your sensor...that's a pretty big heat block! Also, that much meat was probably crowding, if not actually touching, your probe!
When you smoked the mac & cheese, it worked fine, right? Have you ever experienced this on anything else? Next time, do 2 smokes for that much meat, or handle it the way you did, realizing that everything has its limits. That's a whole lotta butt in a #2!
No need to remove the probe and test. You said it was within a degree of the Maverick on the M&C, right? It's fine. Just do a few more reasonably-sized smokes and see if it works like normal.