I've added a custom field to a project to help sort Work packages in views. But after adding the custom field to a view, switching to another page, and coming back to the view the custom field has disappeared. Can you tell me what additional I need to do?

I have the same problem: when I fill in the custome field in a parent I can see it only if I haven't any child related to it.

Try "save"-ing the view after you add the custom field.

1. Add the custom field to your view.
2. Click the hamburger button in the top right of the view (the three dots).
3. Click "Save"

When you complete the "Save" modal, the newly saved View will be visible in the sidebar at left, under either "Public Views" or "Private Views" (depending what you choose in the modal).

Editing the "Default Views" is not possible, as far as I know.