Thank you for contacting us.
There was an known issue concerning the SelectedItem property of the AutoCompleteBox control that was fixed a few weeks ago that could be the cause of this issue.
Could you try to reproduce the issue with our latest internal build and see if it persists, also could you try to reproduce it with another control for example RadComboBox in order to be certain that the issue comes from the AutoCompleteBox control and not the navigation property itself? This would be very helpful in order to narrow down the possible cause of the issue.
Looking forward to your response.
the Telerik team