In a good morning, most try to keep an arch but when heavy weights are used, the back rounds at the bottom.
An arch-back GM is a different exercise, similar to a romanian dl. Set up with a wide stance, arch your back hard, and push your abs out. Now push your butt back. Your range of motion will only be around 8 inches, and you mustn't lose the arch. Keep your chest up and arch up instead of firing through with your hips.