To reproduce:
- Add 200k rows and the call the BestFitColumns method like this (only the visual rows should be measured in this case):
this.radMultiColumnComboBox.BestFitColumns(true, false);

Workaround:
Set the AutoSizeDropDownColumnMode before calling the method:
this.radMultiColumnComboBox.MultiColumnComboBoxElement.AutoSizeDropDownColumnMode = BestFitColumnMode.DisplayedCells;
this.radMultiColumnComboBox.BestFitColumns(true, false);