Trying to book 2 o/w Saver seats for 2/6

MIA - IAH UA 1253
IAH - IAD UA 321
IAD - CHO UA 4856

Website shows only Standard award available despite all three legs showing 2 XN seats available.
UA phone agents also see the XN availability.

BUT UA agents (multiple calls) all say the same thing:
"United looks at the whole routing and even when we add the flights individually it comes out that the routing is not bookable." When putting flights in manually one agent says third flight refuses to show XN and only X "which is only available to non-premiers" (huh?) and can't be booked with the previous two flights in XN.

I have HUCA 3 times and now turning to the ones who really know