It does appear that some transcon 321s have been retrofitted to remove entertainment boxes in lieu of BYOD streaming: https://www.aerolopa.com/aa-airbus-a321t

But per the SeatGuru 32B layout: All window and aisle seats on this aircraft may have restricted storage space and legroom due to the position of an entertainment equipment box.

So it’s very possible that you had the retrofit on the outbound, and the older version on the return.