New Post: checkboxcombo checked box late rendering

I have a checkboxcombo bound to a comma delimited string as follows:

<xc:CheckComboBox  x:Name="_checkComboBox" Margin="5"
             SelectedValue="{Binding SelectedCollectionCodesString}"
             Delimiter=","                                                            
             DisplayMemberPath="Description"     
             ItemsSource="{Binding CollectionCodes}"
             ValueMemberPath="ID"
             IsDropDownOpen="false"
            MaxDropDownHeight="200"/>

The binding works. The comma separated Descriptions appear and the SelectedValue is correctly updated (as comma separated IDs). But the last item selected does not have a checked checkbox unless another item is subsequently selected. Is there a way to fix this behavior?