That's quite unfortunate…

If you're using a mobile pass, the app will very probably take both your inbound/outbound days. With a paper pass (you can buy one from the DB ticket office), you could just fill out the outbound journey and hope they won't complain…

You could ask Snälltåget what you can do. I'd be curious what they say.

Situation is quite clear. You need the travel day of the start and therefore also that outbound day. I think to avoid the (still unfixed ?!) bug of the app, you best add the trip manually

I think you're mixing things up. There's a bug when you're using a night train into your country of residence, where you cross the border after midnight.

I'm not actually sure what happens in the app if you cross the border after midnight using an outbound journey.

Ich hatte genau die selbe Frage in die DB-Community gestellt, da ich das selbe Problem hatte. Die Antwort der DB findest du hier:

Wohnsitzlandreglung Nachtfahrt Interrail (Ausreisetag) | DB Service-Community (bahn.de)

Actually you started talking about that bug, otherwise I forgot for sure (since I was never affected. only tried mobile pass once, never again!).

Obwohl du explizit gefragt hast, sagt die DB dort aber nichts über den Ausreisetag. Ich sehe dort nur eine Aussage über die Nachtzugregelung allgemein :/