To reproduce:

Add a RadMultiColumnComboBox fill it with data and set the following settings:

comboBoxColumnElement.EditorControl.AutoSizeRows = true;
comboBoxColumnElement.EditorControl.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
comboBoxColumnElement.EditorControl.MasterTemplate.AutoGenerateColumns = false;
comboBoxColumnElement.EditorControl.ShowRowHeaderColumn = false;
comboBoxColumnElement.EditorControl.ShowFilteringRow = false;

comboBoxColumnElement.AutoSize = true;
comboBoxColumnElement.AutoFilter = true;
comboBoxColumnElement.AutoSizeMode = Telerik.WinControls.RadAutoSizeMode.FitToAvailableSize;
comboBoxColumnElement.AutoSizeDropDownToBestFit = true;
comboBoxColumnElement.DropDownMinSize = new Size(420, 150);
comboBoxColumnElement.DropDownSizingMode = SizingMode.UpDownAndRightBottom;



When you open the dropdown you will notice that one time the scrollbar will be on the bottom and the next time it will be on the top and so on.

Workaround:

comboBoxColumnElement.PopupOpened += Popup_Opened;....

Timer timer = new Timer();

private void Popup_Opened(object sender, EventArgs e)

{
timer.Tick += Tick;
timer.Interval = 20;
timer.Start();

}

private void Tick(object sender, EventArgs e)
{
comboBoxColumnElement.EditorControl.TableElement.ScrollToRow(comboBoxColumnElement.EditorControl.SelectedRows(0));

        timer.Stop();

}