When directly subclassing QAbstractItemModel, HorizontalHeaderView does not call headerData(). However, by subclassing QAbstractTableModel, it can fetch data correctly.

I quote from the documentation: "If you assign a model that is a QAbstractItemModel, its header data will be used. Otherwise, the data in the model will be used directly (for instance, if you assign a model that's simply an array of strings)."

Here is the result when subclassing from QAbstractItemModel:

And the results when inheriting from QAbstractTableModel:

Please download the attachment and replicate the problem.