Thanks for answering.
I'm using a hierarchical transformation to list folders and their subitems (a custom page type) as modals. I've also limited the view to check only for folders and this custom page type. I'm also pretty sure that no folder is hidden away, as there's only 5 folders.
There's no alternating item transformation, only the ItemTransformation, Separator and Footer (which I'm using to close the last item).
For now, I've limited the Hierarchical view to show only level 0, to list only the folders.
DataItemIndex still returns even numbers tho. 🤔