How to reproduce: check the attached project and screenshots, the correct items are shown in screenshot correct-items-calculated-field.jpg

Workaround: 
public partial class Form1 : Form
{
    private LocalDataSourceProvider provider;

    public Form1()
    {
        InitializeComponent();

        //Setup pivot and add calculated field

        //Workaround
        this.radPivotFieldList1.ValuesControl.CreatingVisualListItem += ValuesControl_CreatingVisualListItem;
    }

    private void ValuesControl_CreatingVisualListItem(object sender, CreatingVisualListItemEventArgs args)
    {
        args.VisualItem = new MyPivotFieldListVisualItem(this.radPivotFieldList1.ViewModel);
    }
}

public class MyPivotFieldListVisualItem : PivotFieldListVisualItem
{
    FieldListViewModel viewModel;

    public MyPivotFieldListVisualItem(FieldListViewModel viewModel)
        : base(viewModel)
    {
        this.viewModel = viewModel;
    }

    protected override void UpdateContextMenu()
    {
        base.UpdateContextMenu();

        PivotFieldListItemButton button = (PivotFieldListItemButton)typeof(PivotFieldListVisualItem).GetField("button", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).GetValue(this);
        string providerName = this.viewModel.DataProvider != null ? this.viewModel.DataProvider.GetType().FullName : String.Empty;
        if (this.Data == null)
        {
            return;
        }

        if (this.Data.DataBoundItem is Value && !(providerName.Contains("Xmla") || providerName.Contains("Adomd")))
        {
            for (int i = 0; i < 5; i++)
            {
                button.Items.RemoveAt(i);
            }
        }
    }
}