Workaround: If possible add the label filters with code
PropertyGroupDescription dateGroupDescription = new PropertyGroupDescription();
dateGroupDescription.PropertyName = "Date";

LabelGroupFilter filter = new LabelGroupFilter();
ComparisonCondition condition = new ComparisonCondition
{
    Condition = Telerik.Pivot.Core.Filtering.Comparison.IsGreaterThan,
    Than = new DateTime(2010, 6, 6)
};
filter.Condition = condition;
dateGroupDescription.GroupFilter = filter;

this.provider.RowGroupDescriptions.Add(dateGroupDescription);