Did you temp them in multiple spots to make sure they were at your target temp? And did they have the texture you are after? At times, the temp isn’t necessarily the true tell, but just checking. But 1 hour per lb at times may just be the rate it cooks.
I have held a brisket the same way, in the 3D for hours, then double wrapped in foil in the cooler with towels, perhaps about the sam length of time, maybe longer.