Steps to reproduce:
1. Run the attached sample application 
2. Enter the value that is not on the list of autocomplete values.
3. The RadAutoCompleteBoxAutomationPeer.Value return null, because the value is not included in SelectedItems collection

If the entered value is included in the list, the RadAutoCompleteBoxAutomationPeer.Value contains a comma (',') in single selection mode.

Workaround: 
1. The first approach is getting the value from nested watermark text box 
2. Another approach is creating own AutomatomationPeer that getting the value from SearchText