Hi Alexis (and thanks Dan for linking this to the Survey123 group!),
You aren't misunderstanding the default value, but hidden fields won't support them like this. What works for what you want is to create a text field with that as a default, that is always hidden through the 'relevant' field. In your case, you can make sure it never turns up by linking it to a select_one question asking for a result that isn't in the choice list. It's a bit of a rough solution, and there's probably a more elegant one, but it works.
