Though not visible at runtime due to type erasure, it should be fixed to keep code maintainers' sanity :-)

From the api doc:

    S - The type of the class contained within the TableView.items list.
    T - The type of the class contained within the TableColumn cells.

That is, S is the type returned by dataFeature.getValue and the class we create a PropertyReference for. The internal implementation incorrectly type-casts that to T to serve the incorrect parameter type (the actual reference takes the actual bean-class, thus is correct)

    @Override
   public ObservableValue<T> call(CellDataFeatures<S,T> param) {
        // would blow without type erasure: the value type is S
        return getCellDataReflectively((T)param.getValue());
    }

    // internal method with incorrect parameter type: rowData
    // is of type S
    private ObservableValue<T> getCellDataReflectively(T rowData) {
          ....
          this.propertyRef = new PropertyReference<T>(rowData.getClass(), getProperty());
     }

Fix: simply remove the type cast and change the internal methods parameter type to S.

While you are at it: I would suggest to change the name columnClass to beanClass because .. that's what it is. This misnomer might have triggered the mix-up: the columnClass indeed is T :-)