This is not uncommon for lanky people. We don't prescribe a specific angle of the back because we know people are built different. The diagnostic points are 1) bar over the middle of the foot and 2) scaps over the bar. You're setup a little too far from the bar for your build. Get a hair closer.
Your first rep setup gets your back pretty flat. What you're probably seeing is the muscle bellies on either side of the spine are bigger than the muscles up the rest of the back. As long as you set the shit out of your back for each rep, and work to keep it locked in place, you'll be fine.
You're also putting the bar down around your knees on the way to the floor, so it ends up even further forward of midfoot for the subsequent reps. Send your ass back and let the bar travel down the legs as you return it and don't bend the knees until the bar gets past them.