Hey Anton
Your upper back isn't set or staying rigid. When we say "point your chest at the ground" that should come from the hinge of the hips going back rather than literally collapsing the upper back and pointing the chest straight down.
When you set up for the rep, lift your chest to get the upper back set and pull the elbows down. Move your eye gaze out to 1-1.5 meters on the floor in front of you (not straight down). Then start the rep by sending your ass back to the wall behind you and keeping your hips away from your feet. The whole torso (including the chest) will lean over so that you can keep your center of mass over midfoot. Think of the upper and lower back as one rigid lever arm. There can't be any bending or softness.
See if you can get the upper back and elbows in the correct position while squatting and check back here.