Maybe your scoring moved?
When doing briskets, I will take 3 toothpicks and insert them "with" the grain on the top.
Note that I use the
Fat Cap Down ( <-Click Link ) method in my verticals, so when I say tooth picks "on top" that is the side I can see the grain.
Anyway, the raw meat has the three toothpicks sticking out, roughly in the same line, following the grain.
But after it cooks, the the two on the ends can be both pointing one way, and the middle pointing the other, meaning the grain started out in approximately the same direction, but ended up in more of a "V" grain wise when finished.
I have had briskets contort to all different forms and the only way to track the grain (and therefore adjust my slicing) was to use the toothpicks as a guide.
Was it possible the "scoring" you did got contorted?
Just trying to help.
The best part is you get to try again!!!
Chop the rest of what was left up into small pieces and chuck into the chili pot!!!