WinRT/C++ TextBox doesn't accept any character input in ContentDialog

Question

I have a desktop program with a WinRT/C++ library (and Microsoft.UI.Xaml.Controls 2.8 prerelease), which in the application will bring up a ContentDialog with a TextBox as its content. However, unfortunately the TextBox does not accept any character input from the keyboard, even though the TextBox is in a focused state. For the experiment so far, the TextBox can only accept input from the Copy and Paste methods. So, is there any workaround or manipulation that can be done to solve this problem?

Code:

TextBox tbox{};

ContentDialog dialog{};
dialog.Title(box_value(L"Title"));
dialog.Content(tbox);
dialog.PrimaryButtonText(L"Create");
dialog.CloseButtonText(L"Cancel");
dialog.XamlRoot(XamlRoot());

auto result = co_await dialog.ShowAsync();

if (result == ContentDialogResult::Primary)
{
    // Create new data from tbox.Text()
}