My experience:
Booked one seat for a friend to ride with me on a road trip.
5/7 SHV - DFW - DCA
8/20 DCA - DFW- SHV
Had to get an early flight out of SHV 5/7 to get a seat but otherwise no problem. The limitation in fact was on the SHV-DFW route, not DFW-DCA based on my search. I was even able to get my friend on the same flight as family for the way back!