HttpContext.User in MVC action is not authenticated, but in Blazor he is

Question

I am developing a Blazor WASM Application and want to export data as a CSV file. The problem is that in the MVC controller action, the user is not authenticated.

Component.razor

<button @onclick="DownloadCsvData">Download CSV</button>

@code {
    void DownloadCsvData()
    {
        NavigationManager.NavigateTo("Export/AsCSV");
    }
}

ExportController.cs

public ExportController : Controller 
{
    public FileStreamResult AsCSV()
    {
        // Why is Request.HttpContext.User.Identity empty 
        // at this point and IsAuthenticated is false? 
        // I need to access user claims here.

        return GenerateCsv();
    }
}

Startup.cs

services.AddHttpContextAccessor();

services
  .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)   
  .AddMicrosoftIdentityWebApi(Configuration.GetSection("AzureAd"));

services.AddControllersWithViews();

app.UseAuthentication();
app.UseAuthorization();

Everywhere else in this application, the user is authenticated as expected after a successfull login. Did I forget something? Do I explicitly do something for enabling the authentication in the controller action?

Just tell me if you need more information or code snippets.