They announced before the flight that they intentionally removed tiles from some areas around the lower flaps specifically to get data on what happens when tiles fail, such as how much burn occurs, how quickly and what type. It appears it was successful in showing varying amounts of burn through and damage in the areas that were intentionally left under-protected.

The part that burned was damaged from early on, likely at, or right after, separation. So the integrity of that flap was already very compromised. I actually thought the flap would disintegrate on re-entry with that kind of damage. But no, they even put full stress on it, unfolding it while supersonic through the atmosphere, like a champ.