It fortunately does happen. As a frequent PC passenger, I don’t like it when my fellow Y pax line up behind the curtain ready to run to the F lav (assuming the cart isn’t blocking the aisle).

In the case of the OP’s mobility challenged dad, some of the PC seats in row 6 and 7 (?) are designated as handicap seats. Anyone can book them, but if a disabled passenger needs them, you might get booted out of your seat. I assume AS is not expecting those row 6/7 disabled passengers to make their way to the very back of the plane for lav visits.