I thought that this was only happening on post May 9th booking.
I travel weekly on SWA for business (as I know most of you do too!) Due to my meeting schedule, I have several contingency city pairs booked on the same date.
I had a round trip AUS->CLE (April 9-12) cancel because I had a CLE->MKE booked on April 12th. This cancelled on 3/27 at midnight. I am unable to book the CLE->AUS fare due to the new "feature" of the reservation system. The round trip was booked before the one way, that's why it was cancelled.
The only recourse that I was told I had was to call customer service and file a complaint.
Please feel free to call 855-234-4654 and also place your complaint too.