I don't think that tubing really anneals, so the heat shouldn't have had that effect, no matter how hot the powder coater got it. In general, the ductility of the metal would depend on the cool down period. If the powder coater just left the frame in the oven to cool, that would lead to annealing -- if it was possible.
I was speculating in the framebuilder's forum that there was built in stress that would tend to push the head tube and lead to buckling. Maybe the head tube reamer took out too much material. Either way, this is a new one.
I was going to say "that'll buff right out," but I decided the owner had been punished enough.