Can't get TryValidateModel() to work

Question

I have a signup action that attempts checks if the model state is valid, attempts a sign in, if fail revalidates the model. I am unable to revalidate the model using TryValidateModel. I have modified some functions to force the model to be invalid.

Here is my sign up action:

[HttpPost]
public IActionResult SignUp(SignUpModel signUp, string returnURL)
{
    if (ModelState.IsValid)
    {
        if (loginManager.SignUp(signUp)) return Ok("Account created");
        TryValidateModel(signUp); // breakpoint 1
    }
    return View(signUp); // breakpoint 3
}

This is the SignUp() function that forces the invalid state:

public bool SignUp(SignUpModel signUp)
{
    signUp.ForceInvalid();
    return false;
}

This is the forced function inside SignUpModel

   internal void ForceInvalid() => Validation.IsDuplicateEmail = true;

Here is my validation attribute class used for email:

class EmailExistsAttribute : ValidationAttribute
{

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (validationContext.ObjectInstance is SignUpModel signUp && signUp.Validation.IsDuplicateEmail)
            new ValidationResult("Email already exists"); // breakpoint 2
        return ValidationResult.Success;
    }

}

I have used the attribute like this:

[EmailExists]
public string Email { get; set; }

The code breaks at the breakpoints in the order: 1 2 3. Since the code breaks at 2, the model state should be invalid. However at breakpoint 3, the ModelState.IsValid is still true and it has no errors. I have even tried ModelState.Clear() even though it doesn't make sense in this context. The output is still the same.