Cannot find User with ID '' after first access from Login to ChangePassword - Identity Asp net core

I've customized the Identity AspNetUsers table by adding a custom field "IsChangedPassword" which, if is false, a user is forced to change the current temp password. In OnPostAsync of Login.cshtml.cs I'v added this lines:

var result = await _signInManager.PasswordSignInAsync(Input.UserName, Input.Password, Input.RememberMe, lockoutOnFailure: false);
                if (result.Succeeded)
                {
                    if (!user.IsChangedPassword)
                    {
                        return RedirectToPage("./Manage/ChangePassword");
                    }
                    else
                    {
                        _logger.LogInformation("User logged in.");
                        return LocalRedirect(returnUrl);
                    }
                }

So, after the first login od an existing user with IsChangedPassword false, nothing goes right:

Unable to load user with ID ''.

My IdentityUser class is:

[Table("AspNetUsers", Schema = "dbo")]    
public class MyCustomUser : IdentityUser
{        
    public bool IsChangedPassword { get; set; }
    public bool IsBlocked { get; set; }
}

And in Program.cs:

	builder.Services.AddMvc();
		builder.Services.AddIdentityCore<MyCustomUser>(options =>
{
	options.SignIn.RequireConfirmedAccount = false;
	// Password settings.
	options.Password.RequireDigit = true;
	options.Password.RequireLowercase = true;
	options.Password.RequireNonAlphanumeric = true;
	options.Password.RequireUppercase = true;
	options.Password.RequiredLength = 6;
	options.Password.RequiredUniqueChars = 6;
	options.Password.RequiredUniqueChars = 6;
	// Lockout settings.
	options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(5);
	options.Lockout.MaxFailedAccessAttempts = 5;
	options.Lockout.AllowedForNewUsers = true;
	// User settings.
	options.User.AllowedUserNameCharacters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_";
	options.User.RequireUniqueEmail = true;
})
.AddRoles<MyCustomRoles>()
.AddSignInManager<SignInManager<MyCustomUser>>()
.AddDefaultTokenProviders()
.AddEntityFrameworkStores<MyDBIdentityContext>();

Why I cannot force to correctly change password at first access? The user EXISTS; IsChangedPassword is false; I didn't change anything in ChangePassword.cshtml.cs.

Thank you in advance.

I built an example, but your problem does not appear. You can refer to my steps below.

Right click on project -> Add -> New Scaffolded Item -> Identity, then select the corresponding layout, the page to be changed, the database context and the model:

After installing Identity, add your custom properties to MyCustomUser:

public class MyCustomUser : IdentityUser
{
    public bool IsChangedPassword { get; set; }
    public bool IsBlocked { get; set; }
}

Migrate and update the database:

Add-Migration CreateIdentitySchema
Update-Database

Add the corresponding partial view in Layout.cshtml, and add the RazorPage middleware in Program.cs:

<partial name="_LoginPartial" />

app.MapRazorPages();

Inject UserManager in Login.cshtml.cs and add judgment conditions after successful login:

public class LoginModel : PageModel
{
    private readonly SignInManager<MyCustomUser> _signInManager;
    private readonly UserManager<MyCustomUser> _userManager;
    private readonly ILogger<LoginModel> _logger;

    public LoginModel(SignInManager<MyCustomUser> signInManager, ILogger<LoginModel> logger, UserManager<MyCustomUser> userManager)
    {
        _signInManager = signInManager;
        _logger = logger;
        _userManager = userManager;
    }
    //...
    public async Task<IActionResult> OnPostAsync(string returnUrl = null)
    {
        //...
        var result = await _signInManager.PasswordSignInAsync(Input.Email, Input.Password, Input.RememberMe, lockoutOnFailure: false);
        if (result.Succeeded)
        {
            var user =await _userManager.FindByEmailAsync(Input.Email);
            if (!user.IsChangedPassword)
            {
                return RedirectToPage("./Manage/ChangePassword");
            }
            else
            {
                _logger.LogInformation("User logged in.");
                return LocalRedirect(returnUrl);
            }
        }
        //...
    }
}

Then in ChangePassword.cshtml.cs, add the following code after changing the password successfully:

user.IsChangedPassword = true;
await _userManager.UpdateAsync(user);

You can check if any of your steps is incorrect.