AutoCompleteBox binding text disappear when using ShowAsPopupAsync

With ShowWindowDialogAsync or ShowAsync, it's fine. But with ShowAsPopupAsync, it somehow makes the AutoCompleteBox text disappear. Am I missing something?
```
      <AutoCompleteBox
        FilterMode="Contains"
        ItemsSource="{Binding EmployerSource}"
        Text="{Binding Item.Employer}" />
```
```
await msgbox.ShowAsPopupAsync(win)
```