I've had that a couple times. Booking a P/Z ticket through UA where the first and last legs were AC (Jazz without J seating), the first leg gets booked into a Tango fare, no issues with UA of course, last AC Jazz leg gets booked into full Y. With the last segment being Y, AC will let me select a preferred seat no charge, but for the first leg being booked into Tango and not having SE status I can't select a seat at all. As is frequently mentioned, AC doesn't even have the capability of taking my money I want to pay to select a seat on this Tango fare.