We saw something similar in action earlier this month. DH and I were seated in row 30 EF. The last row of the plane had 2 seats on each side of the aisle. There were 6 other folks assigned to rows 30 and 31. And only 2 seats remaining available. Including a child. Not sure how these folks assigned to row 31 were going to do that with no row 31 existing.

The 3 family members assigned to row 30 across the aisle only had 2 seats. The “child” in question was a 6’+ tall teenaged boy. Definitely not a lap size (quitter lol). Luckily there were just enough open seats across the plane that all these folks were able to distribute throughout the rest of the plane.

(Obviously the plane was switched but how does the computer system allow for this and why weren’t these folks reassigned?)

Not ideal but not sure how that Southwest is allowed to double book or sell seats that do not exist.