Those center vents are totally manually operated. There is no motorized mechanism to move them other than by sliding the wheels that open and close them. So, if they are closing on their own at higher fan speeds, either some other vent tubing to the floor, side dash vents or the windshield are partially blocked. You say your driver's side floor vent air is weak? Maybe in that vent tubing is your problem.
The brushed aluminum look plastic surround that surrounds those center vents, the radio, and the HVAC, just pries right out of the dash on all sides. Use a flat screw driver blade wrapped in tape (to not scratch anything) and pry along its edges until totally free and pull it straight out. (Google "2013 outback dash vent pictures" to see what I mean.)
At that point you can get to the wheel mechanisms that close those center vents and possibly put some tape under the wheels to add friction, so they won't blow open at the higher fan speeds.
Then again, maybe some floor carpeting is covering the driver's side floor vent up by the center dash/console area. It was in my 2007 OutBack LTD. That one was an easy fix for me.
If neither solves the issue, you may have to tear the center console/dash apart to get at the HVAC system and its tubing to the various vents, and check out the various door actuators that direct the air. Its not hard to do, just very time consuming. Take lots of pictures if you do, in order to remember where everything goes and the order. Been there, done that chasing down a mouse nest in the HVAC air box years ago and two OutBack's ago. Good Luck!