.NET Android App can read textfile in Documents only if created by itself

Question

I'm writing an Android app in .NET using Avalonia. I'm attempting to read a textfile from the Documents folder, but am hitting an odd behavior: I can read the file only if it was created by the app itself, and in no other case. Below is an example with comments to explain:

private async void test_command()
{
    // Use Xamarin.Essentials to get READ_EXTERNAL_STORAGE & WRITE_EXTERNAL_STORAGE
    // The permissions are also listed in the manifest.
    // This succeeds, shows the prompt, & says "granted"
    var hasRead = await Permissions.RequestAsync<Permissions.StorageRead>();
    var hasWrite = await Permissions.RequestAsync<Permissions.StorageWrite>();
    if (hasWrite != PermissionStatus.Granted || hasRead != PermissionStatus.Granted)
    {
        show_notification_warning("Storage Permission denied");
        return;
    }

    var dirname = "/storage/emulated/0/Documents/";
    var defaultFilename = "test.txt";
    var content = "This is some sample content";

    // Ask for a filename & write some text
    // (the dialog is handled by HanumanInstitute.MvvmDialogs).
    // This works.
    var promptVm = _dialogService.CreateViewModel<TextPromptViewModel>();
    promptVm.Title = "Enter filename to WRITE";
    promptVm.Text = defaultFilename;
    var success = await _dialogService.ShowDialogAsync<TextPromptWindow>(this, promptVm).ConfigureAwait(true);
    if (success.Value)
    {
        var writeFilename = dirname + promptVm.Text;
        using (var writer = System.IO.File.CreateText(writeFilename))
        {
            await writer.WriteLineAsync(content);
        }
    }

    // Now ask for another filename to read.
    // If I read back the file I just created, it works.
    // However, if I enter the name of a file that I copied into the device
    // over USB, or created with a text editor app, it throws 
    // System.UnauthorizedAccessException: Access to the path '/storage/emulated/0/Documents/test2.txt' is denied.
    promptVm = _dialogService.CreateViewModel<TextPromptViewModel>();
    promptVm.Title = "Enter filename to READ";
    promptVm.Text = defaultFilename;
    success = await _dialogService.ShowDialogAsync<TextPromptWindow>(this, promptVm).ConfigureAwait(true);
    if (success.Value)
    {
        var readFilename = dirname + promptVm.Text;
        using (var reader = System.IO.File.OpenText(readFilename))
        {
            var readBack = await reader.ReadToEndAsync();
        }
    }
}

At first it looks like a file permissions issue, but I've checked both the permissions & the owner and they're identical. i.e. if I have Documents/test1.txt and Documents/test2.txt, where one was created by the app per above (and can be read) and the other was copied onto the device or created by a texteditor app (and throws), both show the same permissions and the same owner.

Why won't Android let me open a textfile that I've copied into the Document folder or created with another app?