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