Pullman Wholemeal Tangzhong Loaf-Concaved and Sagged

Hi! It is my first time using a Pullman pan, and I used this recipe. http://en.christinesrecipes.com/2012/10/tangzhong-wholemeal-loaf.html

I used Bluebird White Wheat Flour that I ground myself. I measured out 150 grams of the fresh ground flour like the recipe says. I followed all the directions exactly, and I used the bread machine to mix the dough and do the first proofing. For the final rise, I had the bread rising in the Pullman loaf pan on top of the stove that was heating up. I baked the bread for 30 minutes with the top on. I immediately turned the bread out, and it looked beautiful, tall and perfect. Then it started singing into itself. The ends kind of concaved into the loaf and look like octagons, and the loaf sunk a bit too! It was slightly undercooked too when I sliced a end. I fixed that by shoving it back into the oven for 5 minutes; however, the aesthetics of my bread suffered. If I cooked the bread the full 35 or maybe 40 minutes, does anyone think the bread will not shrink into itself? Or do I need to try something else, so it will keep it's symmetrical shape like the recipe's picture does?

Thanks for any help and advice you can give me.