To reproduce:
- Open the attached project.
- Type a text so there are no results in the drop down.
- Press Enter.
- Click the arrow button.

Workaround: 
private void RadMultiColumnComboBox1_DropDownOpened1(object sender, EventArgs e)
{
    if (radMultiColumnComboBox1.Text == "")
    {
        radMultiColumnComboBox1.EditorControl.FilterDescriptors.Clear();
        CompositeFilterDescriptor compositeFilter = new CompositeFilterDescriptor();
        compositeFilter.FilterDescriptors.Add(new FilterDescriptor("Drug", FilterOperator.StartsWith, ""));
        compositeFilter.FilterDescriptors.Add(new FilterDescriptor("Name", FilterOperator.StartsWith, ""));
        compositeFilter.LogicalOperator = FilterLogicalOperator.Or;
        radMultiColumnComboBox1.EditorControl.FilterDescriptors.Add(compositeFilter);
    }
}