What went wrong ?? You didn't cook it long enough, you are trying to bring temperatures in to the mix, you didn't follow the rules for success. The only place that need to probe tender on a brisket is the place that you failed to let cook until it was probe tender.
So what did you learn >you learned that the probe test is the last word.
you learned where to locate the only place on the brisket that has to be probe tender, if that spot is tender the rest will be too.
BBQ RULES FOR SUCCESS
YOU CAN NOT COOK GREAT BBQ ON A CONSISTENT BASIS BY COOKING TO AN INTERNAL TEMPERATURE OR BY TIME ( XXX MIN PER LB) YOU MUST COOK BY FEEL! For Brisket it must pass the poke test(probe like soft butter in the thickest part of the Flat) Ribs pass the Bend Test, Pork Butts when the bone wiggles loose. These are the only reliable methods to ensure that your cook will be a success. There is one exception to these rules and that is Poultry which must achieve and internal temp of 170 deg in the thickest part of the thigh and 165 in the breast.
I'm a Proxy Vegetarian> Cows eat grass & I eat cows.