It is not necessary, you can simply write down/choose actual train when you stand on platform awaiting it, Essentially same as old-style paper pass -when I sit I take it out and write it-yes, I am aware this is to the rules a few secs too late.

In the mobile pass, trains added from the planner can't overlap but you can always add a manual journey and then trains can overlap. I do that when I can get a delayed train that would normally have left before my previous train arrived.

On an iPhone though, don't add manual journeys before there's confirmation that the bug has been solved that makes journeys disappear after adding a manual journey.