To reproduce:
- Use the mouse to select an item from the drop down list.
- Using the keyboard, type a letter  and use the mouse to select one more item.
- Change the DataSource with a button.

Workaround:
radCheckedDropDownList1.DataSource = null;
radCheckedDropDownList1.AutoCompleteMode = AutoCompleteMode.None;

radCheckedDropDownList1.ValueMember = "Id";
radCheckedDropDownList1.DisplayMember = "Value";
radCheckedDropDownList1.CheckedMember = "IsSelected";
radCheckedDropDownList1.DataSource = list;

radCheckedDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;