Modelstate validation failed unexpectedly even with proper data passed from view

Change your middleware like below can fix the issue.

Here is the test result

RequestLoggingMiddleware.cs

namespace _79436884
{
    public class RequestLoggingMiddleware
    {
        private readonly RequestDelegate _next;

        public RequestLoggingMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task Invoke(HttpContext httpContext)
        {
            var originalBodyStream = httpContext.Response.Body;

            try
            {
                httpContext.Request.EnableBuffering();

                var requestBody = await new StreamReader(httpContext.Request.Body).ReadToEndAsync();
                httpContext.Request.Body.Position = 0; // Key fix: reset stream position

                using (var responseBody = new MemoryStream())
                {
                    httpContext.Response.Body = responseBody;

                    await _next(httpContext);

                    responseBody.Position = 0;
                    var responseContent = await new StreamReader(responseBody).ReadToEndAsync();
                    responseBody.Position = 0;
                    await responseBody.CopyToAsync(originalBodyStream);
                }
            }
            finally
            {
                httpContext.Response.Body = originalBodyStream;
            }
        }
    }
}

Here is my detailed steps.

1. HomeController.cs

using _79436884.Models;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using System.Diagnostics;

namespace _79436884.Controllers
{
    public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;

        public HomeController(ILogger<HomeController> logger)
        {
            _logger = logger;
        }

        [HttpGet]
        [AllowAnonymous]
        public IActionResult Authenticate()
        {
            return View();
        }

        [HttpPost]
        public async Task<IActionResult> Authenticate(AuthenticateViewModel model)
        {
            if (!ModelState.IsValid)
            {
                ViewBag.ErrorMessage = "Validation failed";
                return View(model);
            }

            await Task.Delay(100);
            return Json(new { success = true });
        }

        public IActionResult Index()
        {
            return View();
        }

        public IActionResult Privacy()
        {
            return View();
        }

        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }
    }
}

2. AuthenticateViewModel.cs

using System.ComponentModel.DataAnnotations;

namespace _79436884.Models
{
    public class AuthenticateViewModel
    {
        [Required]
        public string? Email { get; set; }

        [Required]
        [DataType(DataType.Password)]
        public string? Password { get; set; }
    }
}

3. Register the middleware.

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Home/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseMiddleware<RequestLoggingMiddleware>();

app.UseAuthorization();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Authenticate}/{id?}");

app.Run();

4. Authenticate.cshtml

@model _79436884.Models.AuthenticateViewModel

<h2>Login</h2>
<form id="form-submit" asp-action="Authenticate" method="post">
    <div class="form-group">
        <label asp-for="Email"></label>
        <input asp-for="Email" class="form-control" />
        <span asp-validation-for="Email" class="text-danger"></span>
    </div>

    <div class="form-group">
        <label asp-for="Password"></label>
        <input asp-for="Password" type="password" class="form-control" />
        <span asp-validation-for="Password" class="text-danger"></span>
    </div>

    <button type="submit" class="btn btn-primary">Submit</button>
</form>

<div id="result" class="mt-3"></div>

@section Scripts {
    <script>
        $(function () {
            $('#form-submit').submit(function (e) {
                e.preventDefault();
                var formData = $(this).serialize();
                $.ajax({
                    url: $(this).attr('action'),
                    type: "POST",
                    data: formData,
                    success: function (response) {
                        $('#result').html('<div class="alert alert-success">Success!</div>');
                    },
                    error: function (xhr) {
                        var errorMessage = xhr.responseJSON?.errorMessage || "Validation failed";
                        $('#result').html(`<div class="alert alert-danger">${errorMessage}</div>`);
                    }
                });
            });
        });
    </script>
}