Use attached to reproduce:

- Press button1, open the drop-down and use the mouse wheel.

Workaround:

class MyMultiColumnComboPopupForm : MultiColumnComboPopupForm
{
    public MyMultiColumnComboPopupForm(PopupEditorBaseElement owner)
       : base(owner)
    {


    }
    public override bool OnMouseWheel(Control target, int delta)
    {
        return true;
    }
}
class MyMCCB : RadMultiColumnComboBox
{
    protected override RadMultiColumnComboBoxElement CreateMultiColumnComboBoxElement()
    {
        return new MyMCCBElement();
    }
}
class MyMCCBElement : RadMultiColumnComboBoxElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadMultiColumnComboBoxElement);
        }
    }
    protected override RadPopupControlBase CreatePopupForm()
    {
        var popupForm = new MyMultiColumnComboPopupForm(this);
        popupForm.EditorControl.Focusable = false;
        popupForm.MinimumSize = this.DropDownMaxSize;
        popupForm.MaximumSize = this.DropDownMaxSize;
        popupForm.Height = this.DropDownHeight;
        popupForm.VerticalAlignmentCorrectionMode = AlignmentCorrectionMode.SnapToOuterEdges;
        popupForm.HorizontalAlignmentCorrectionMode = AlignmentCorrectionMode.Smooth;
        popupForm.RightToLeft = this.RightToLeft ? System.Windows.Forms.RightToLeft.Yes : System.Windows.Forms.RightToLeft.Inherit;
        this.WirePopupFormEvents(popupForm);
        return popupForm;
    }
}