1) Were you measuring temp based on door temp or other temp device ?
2) Was the brisket close to the upper wall getting the radiant heat ?
Reason I ask, that as a former owner of medium spicewine, I know these two variables can impact estimated time for doneness.
Lonestar 24" Vertical offset w/ Insulated Firebox & Performer Platinum
Former owner: Jambo Backyard, Klose BYC, Med. Spicewine, Pitts & Spitts, XL & (2) Med. BGE's, 22" WSM & (2) 18" WSM's, 18" & 22" Weber Kettles