This is overproofed. Here's how I know:
The scores collapsed inward instead of opening up. Look at where the blade cut. It's sunken, not lifted. That's the clearest sign. When dough is properly proofed, those scores open cleanly and create an ear. When it's overproofed, the structure is too weak to hold, so everything falls. The loaves spread out instead of up. Overproofed dough doesn't have the strength to rise in the oven. It just slumps. How to avoid this: Use the poke test. Before you score and bake, gently poke the shaped dough with a floured finger. - Underproofed: Springs all the way back immediately. Give it more time. - Perfect: Springs back slowly about halfway. Bake it now. - Overproofed: Doesn't spring back at all. The indent stays. You've gone too far. For banh mi, you're shaping tight baguettes and doing a final proof. Check them every 15-20 minutes toward the end. Don't guess. Poke them. The dough will tell you when it's ready. Listen to it. She will nail this next time.