ModelState.IsValid() does not validate even after the previous ModelState was closed - asp.net-core

Question

I am using an if statement which should perform some operations when the model state is valid, but apparently ModelState.IsValid does not return true. I have tried clearing the previous ModelState using ModelState.Clear() but it did not work as well.
Here is my Controller
// This is the first action for which I'm using the model state
[HttpPost]
public async Task<IActionResult> Register(userModel model)
{
if (ModelState.IsValid)
{
userData lcl_userr = new userData
{
username = model.username,
password = model.password,
email = model.email
};
_context.users.Add(lcl_userr);
await _context.SaveChangesAsync();
ViewBag.Success = "Registered successfully";
ModelState.Clear();
return View();
}
return View(model);
}
// And this is the second one
[HttpPost]
public IActionResult Index(userModel model)
{
if (ModelState.IsValid)
{
var findValue = _context.users.Any(o => o.username == model.username);
var findValue2 = _context.users.Any(o => o.password == model.password);
if (findValue && findValue2)
ViewBag.Name = "Success";
}
return View(model);
}
And here's the form
<form method="post" asp-action="Index" asp-controller="Portal">
<div class="text-danger"></div>
<div class="text-warning">#ViewBag.Name</div>
<div class="form-group">
<label class="mt-4 asp-for="username"">Username</label>
<input class="form-control" type="text" required="required" asp-for="username" />
<span></span>
</div>
<div class="form-group">
<label class="mt-4" asp-for="password">Password</label>
<input type="password" class="form-control" required="required" asp-for="password"/>
<span></span>
</div>
<center>Don't have an account? <a asp-controller="Portal" asp-action="Register">Register here</a>.</center>
<center><button value="login" class="btn btn-primary mt-3 w-25 mb-3 align-content-center">Login</button></center>
</form>
The model class being called inside the action and the class members work fine and the values it gets from the form are also working fine, it's just the model state which does not work.
The userModel class
public class userModel
{
[Display(Name = "username")]
[Required]
public string username { get; set; }
[Required]
[Display(Name = "password")]
public string password { get; set; }
[Required]
[Display(Name = "email")]
public string email { get; set; }
}

Answer 1

The second request to public IActionResult Index(userModel model) whose model doesn't contais email. IsValid is false.
[Required]
[Display(Name = "email")]
public string email { get; set; }
The ModelState instance for the email address has an error in the Errors collection.
When MVC creates the model state for the submitted properties, it also goes through each property in the ViewModel and validates the property using attributes associated to it. If any errors are found, they are added to the Errors collection in the property's ModelState.
Solution
Just reomve if (ModelState.IsValid) in Index action and there is no need to add it at all.
or if you insist to keep if (ModelState.IsValid), you need to add email inside form.
<input type="hidden" asp-for="email" />