Some reason, I have my rack set to 31" as well, but setting these rails up exactly as you have, my l4500 still sticks out about 2-3"
I’ve even tried adjust my outer rails to be way back almost flush with the rear “L” brackets and its still producing the same results. I believe something is hitting aomewhere.
May have to investigate further this weekend.
EDIT: Figured out my issue, was the shelf above sagging just enough that the rosewill was hitting the back l brackets preventing it from going back all the way.