i.m.ui.contentapp.action.OpenDetailSubappActionDefinition#viewType is an opaque string. It's semantics and default value is not evident.

According to i.m.ui.contentapp.ResolveJcrNodeFromLocationContext#resolveNode permissible values are add, edit, view, version and the empty string. The latter is the default and results in the fields in the detail subApp not being populated.

As discussed on Slack this area might undergo further changes in the future. In the meanwhile I suggest to add a doc comment for the current behaviour and change the default value to view.