@sierdzio At which point do these functions get called so I can debug the issue?

They're called all the time. When view is initialised, when mouse hovers over the view, when focus changes - really they are typically called hundreds of times whenever you do anything in the app.

I've treid looking trough the source of QTreeView but don't see it anywhere. Now I just have a break point at those functions but they don't seem to get called from the base level items, only the root item.

The view starts at root item because initially it does not know anything more. So first, it will check index(QModelIndex()) to get the root item (in QTreeView, the root is invisible!), then it will check if root has children, then it will call index() for all items, and it will call data() for all roles to get all data that needs to be displayed. Then rinse and repeat for all children, their children etc.

I was expecting the 'index' or 'hasChildren' to be from one of the base level items during initialisation but it does not seem to be the case.

In your model, both hasChildren() and rowCount() need to return correct data for the hierarchy to be shown correctly in QTreeView. And yes, code in index() needs to be correct as well.