Your middle seating position has a shoulderbelt, correct?
You lock the belt by pulling the shoulderbelt all the way out, correct?
It's hard to tell by the pictures what, exactly, the problem could be, but I'm thinking if your belts lock at the retractor, you may just be pulling up on the shoulderbelt too much as you lock it/after you lock it. That can cause tilt with both RF and FF seats. I make sure to lock the shoulderbelt last; tighten the lapbelt portion by pulling hard on the shoulderbelt portion, but pull parallel to the lapbelt, not up at an angle (which feels more natural, but tends to cause tilt). Make sure the lapbelt portion is tight, then lock the shoulderbelt and remove any slack in it, but not pulling up on the shoulderbelt where it will cause tilt. The idea is to make the lapbelt tight enough to hold the carseat tight; locking the shoulderbelt only ensures the lap portion doesn't loosen up. Also make sure, as you are tightening the belt, that you are placing pressure on the carseat evenly, not more or less on one side or the other.
How old/tall/heavy is your little one?
In general, a slight tilt is not a safety concern, as long as the seat is, indeed, installed tightly. It would still drive me batty, though, and I'd try to fix it.