The issue seems to be with the TPE trains; as they connect well with the train to Shildon the journey planner only shows itineraries including them, not earlier trains (XC, LNER) with a longer connection nor later trains that miss a connection.
As TPE have horrendous levels of cancellations you're better off booking LNER to Darlington (£9 ish without a railcard if bought in advance) then the last bit separately. Alternatively head over to trainsplit.com and under advanced options choose to exclude Transpennine Express; there are probably some LNER & Connections advances available at not much more than the York - Darlington fare.
(The 17th is inbetween two strike dates and TPE aren't apparently confident yet they can run the trains in the timetable, so no reservations released/available for them).