Why is row 21 paid on one side but free on the other on the MAX 8?

Looking at row 21 on the 737 MAX 8 and it makes no sense. Port side is a paid seat, starboard side is free to select. AeroLOPA doesn’t show any real difference between the two sides. Same pitch, same location, nothing obvious.

Is this just another case of AA’s pricing logic being weird, or is there something subtle I’m missing like a blocked recline or equipment difference?