You do not suffer from butt wink, which is a pretty awful term, but lumbar overextension followed by lumbar flexion. At the top of the squat, you throw your head back and overextend your low back. As you get lower, your low back rounds while you are still looking straight ahead. As you ascend, you send your butt and knees back, followed by extending your hips. You look a little like a towel that someone is trying to snap.
Look downward for the whole set. Chin down, chest up. Get tighter. Lock your back and torso down with a big breath and don't let it move. Do not allow yourself to relax at the bottom.