Bread cracks in the oven
Your picture makes me want to say that it's a classic case of underproofing, but... You also mixed that dough at a pretty high speed. I think you may have over-developed the gluten. Sometimes if the gluten is too tight, it won't be stretchy enough to accommodate proofing and oven spring, which can lead to a rough surface during proofing and ripping while baking. This is more frequently a problem for buns since their shape requires them to be molded tighter than a loaf would normally be, and their small size means that they rise faster than the gluten can relax. You might have better luck only mixing for the recommended time on the lower speed, and then folding it once or twice during the rising time.