I have an ASPXGridview that includes a GridViewDataComboBoxColumn with two columns and an EnitityServerModeDataSource. See the source from the aspx and the aspx.cs.

ASPX:

<dx:GridViewDataComboBoxColumn FieldName="ServiceCode" VisibleIndex="3"  >
    <PropertiesComboBox IncrementalFilteringMode="Contains" DataSourceID="EntityServerModeDataSourceBillingService"
        TextFormatString="{0}" ValueField="ServiceCode" 
        ValueType="System.String">                    
        <ValidationSettings RequiredField-IsRequired="true" RequiredField-ErrorText="Service Code is required!">
        </ValidationSettings>
        <Columns>
            <dx:ListBoxColumn FieldName="ServiceCode" Caption="Service" Width="120"/>
            <dx:ListBoxColumn FieldName="Description" Width="400" />
        </Columns>                    
    </PropertiesComboBox>
</dx:GridViewDataComboBoxColumn>

<dx:EntityServerModeDataSource ID="EntityServerModeDataSourceBillingService" runat="server" />

ASPX.CS:

protected void Page_Init(object sender, EventArgs e)
{
    ...
    EntityServerModeDataSourceBillingService.Selecting += EntityServerModeDataSourceBillingService_Selecting;
} 

private void EntityServerModeDataSourceBillingService_Selecting(object sender, LinqServerModeDataSourceSelectEventArgs e)
{
    var visibleFieldList = new HashSet<string>();
    visibleFieldList.Add("ServiceCode");
    visibleFieldList.Add("Description");

    e.KeyExpression = nameof(BillingDAL.BillingService.ServiceCode);
    var query = _billingServiceRepository.GetAll();
    e.QueryableSource = query.Select(string.Format("new ({0})", visibleFieldList.ToSeparatedString()));
}

It works like a charm, including the filtering.
But while the DataSource contains 118 elements, only 100 are shown.
What am I overlooking?