When you add a Data Detail element to a page/modal it does not show in Design view (no icons to add elements. It’s always been that way.). But now, in the App Connect Panel the insert into panel is blank.
The only way to add the first element into a Data Detail is to use the App Connect panel and add your first element either before or after the data detail and then drag it inside. Once you do that you can use that element to add more elements as expected.