You should not use both function SelectedText and SelectedItem in autocomplete. it's a bug of AutoCompleteBox..... A better way is to set the visiblity of the textbox and AutoCompleteBox on GotFocus and LossFocus. This Way You Will Defiantly Solve You Problem
private void DemoAutoComplete_LostFocus(object sender, RoutedEventArgs e)
{
DemoTextBox.Visibility = Visibility.Visible;
DemoAutoComplete.Visibility = Visibility.Collapsed;
DemoTextBox.Text = OCRAutoComplete.Text;
((DemoVM)this.DataContext).SelectedDemoText = DemoAutoComplete.Text;
}
private void DemoTextBox_GotFocus(object sender, RoutedEventArgs e)
{
DemoAutoComplete.Text = OctTextBox.Text;
DemoTextBox.Visibility = Visibility.Collapsed;
DemoAutoComplete.Visibility = Visibility.Visible;
DemoAutoComplete.Focus();
}