It looked like plasma got between the flap and the body. I wonder if that means something broke/melted to allow that, or if the design just allowed it accidentally.
They've been concerned about burn through in that area for a while, but they didn't get to test it before now to understand how it'd perform in reality. IIRC they were even calling out that they were surprised that the temperature readings in other parts were in good agreement with the simulations, which is probably indicative of the limited confidence they had for that part.
It looked like the flap was starting to glow internally in the middle, right before the burn-through on the hinge point. I wonder if it maybe had a lost tile on the other side that evolved into the burn-through we saw in the video.
I imagine this is what happened to Columbia
Part of the build up said that they had deliberately weakened / thinned some of the tiles in order to test what the tolerance was. It seems that they must have gotten some incredible data about the mode of failure.
That was for the base of the ship, so that they could add more sensors.
Survivorship bias!
Hopefully the takeaway is something straightforward, like thicker shielding in one area and not a big redesign of the flaps
They already do have some pretty significant flap redesigns in the pipeline. Slightly smaller and placed slightly farther back from the centerline.