To reproduce:
Type "Th" into the multi-column combo box in the attached project.  Then press and hold the Backspace key until "Th" is deleted.  Then release Backspace key.  There remain only the filtered options in the dropdown.
If I press and release the backspace key 1x for each typed letter, then the dropdown repopulates as expected.

Workaround: 
 private void radMultiColumnComboBox1_TextChanged(object sender, EventArgs e)
        {
            this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ApplyFilter();
        }