Should the open journey be closed at a station further down the line after Ally Pally to show that your new ticket (purchased on line) was to be used from that further station, hence you had boarded a train at Ally Pally with a valid ticket (the contactless one). If you try to close your journey at Ally Pally (as you had originally intended), then you may have no defence of boarding a train without a valid ticket, even though you had bought a ticket before the train left.(and for cleanliness you could probably get the open journey completed consistent with that, but technically what happens after-the-fact is irrelevant).