i have always had trouble with flat only brisket. a cooked flat most likely doesn't have the mass to keep itself moist and tender, like a whole packer would. both during cooking and resting.
but, possibly, did you vent the brisket before resting in a cooler? if not it may have overcooked itself because it was too hot.
i have coolered whole packers as well for six hours and haven't had issues.
good luck and have fun with your 4th cook!