Is the Application associated with an Applicant? If not, the nested data view will be empty and read only. If you want to show an Applicant in the nested data view, you could either create an Applicant upfront, and point the Application to it, or create a microflow data source for the nested data view to check whether the Application already has an Applicant, and if not create one and associate it with the Application.
Got it, it works as expected. I just had to invoke a Workflow and create a blank Applicant and associate it with the application.
I expected it to create a blank one when I selected the nested data view as editable though. Anyway, that is how the tool is and we use it that way.