In this particular scenario, the RadDropDownListEditor element inside the RadComboBoxColumn cell contains an empty string item. So when we try to clear the value of the cell and leave it empty, the internal logic will return the last selected valid value. The following code demonstrate a simple example:

DataTable dt = new DataTable();
dt.Columns.Add("id", typeof(int));
dt.Columns.Add("codice", typeof(string));
dt.Columns.Add("nome", typeof(string));

dt.Rows.Add(0, "", "");
dt.Rows.Add(1, "I622", "Seravezza");
dt.Rows.Add(2, "G628", "Pietrasanta");
dt.Rows.Add(3, "L833", "Viareggio");

The first row in the DataTable which the RadGridView contains an empty value. If for example, the second row is selected and we use Backspace/Delete to clear the text and move to another cell. The selected item won't be changed. The cell value won't be cleared. To workaround this we could avoid using empty strings. We could use space or some other symbol to allow the user to clear the value from the drop-down popup.

dt.Rows.Add(0, "", "-");
OR
dt.Rows.Add(0, "", " ");