WriteAsync error. Cannot write plain text

Question

I have problem with WriteAsync method in asp.net core.

I have code:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.Use(async (HttpContext context, RequestDelegate next) =>
{
    await context.Response.WriteAsync("Hello");
    await next(context);
});

app.Use(async (HttpContext context, RequestDelegate next) =>
{
    await context.Response.WriteAsync("Hello again");
    await next(context);
});

app.Run();

It runs into following error:

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.InvalidOperationException: StatusCode cannot be set because the response has already started.
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ThrowResponseAlreadyStartedException(String value)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.Microsoft.AspNetCore.Http.Features.IHttpResponseFeature.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Http.DefaultHttpResponse.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Builder.ApplicationBuilder.<>c.<Build>b__18_0(HttpContext context)
         at Program.<>c.<<<Main>$>b__0_1>d.MoveNext() in C:\aspnetApps\WebApplication_test\WebApplication_test\Program.cs:line 13
      --- End of stack trace from previous location ---
         at Program.<>c.<<<Main>$>b__0_0>d.MoveNext() in C:\aspnetApps\WebApplication_test\WebApplication_test\Program.cs:line 7
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
warn: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[2]
      The response has already started, the error page middleware will not be executed.
fail: Microsoft.AspNetCore.Server.Kestrel[13]
      Connection id "0HMN0TEI6NPN9", Request id "0HMN0TEI6NPN9:00000002": An unhandled exception was thrown by the application.
      System.InvalidOperationException: StatusCode cannot be set because the response has already started.
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ThrowResponseAlreadyStartedException(String value)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.Microsoft.AspNetCore.Http.Features.IHttpResponseFeature.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Http.DefaultHttpResponse.set_StatusCode(Int32 value)
         at Microsoft.AspNetCore.Builder.ApplicationBuilder.<>c.<Build>b__18_0(HttpContext context)
         at Program.<>c.<<<Main>$>b__0_1>d.MoveNext() in C:\aspnetApps\WebApplication_test\WebApplication_test\Program.cs:line 13
      --- End of stack trace from previous location ---
         at Program.<>c.<<<Main>$>b__0_0>d.MoveNext() in C:\aspnetApps\WebApplication_test\WebApplication_test\Program.cs:line 7
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
         at Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.ExecuteWithFilterAsync(IHttpSocketAdapter injectScriptSocket, String requestId, HttpContext httpContext)
         at Microsoft.AspNetCore.Watch.BrowserRefresh.BrowserRefreshMiddleware.InvokeAsync(HttpContext context)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)

If i add tags to hello messages, the app runs correctly. Like this:

await context.Response.WriteAsync("<p>Hello</p>");

If i change content-type to 'text/plain' it runs into the same error. Like this:

app.Use(async (HttpContext context, RequestDelegate next) =>
{
    context.Response.ContentType = "text/plain";
    await context.Response.WriteAsync("Hello");
    await next(context);
});

I have no idea why is it working like this. I`d like to reply by plain text.