Custom Validation not firing, build-in does

Question

I cant get my Custom Validator to fire in ASP.NET Core MVC.

I try to use a validator to check if the domain of the input e-mail is known

public class AllowedDomain : ValidationAttribute
{
    public AllowedDomain() : base()
    {
    }

    public string GetErrorMessage() => $"Some Error Message";

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        AllowedDomainRepository repo = (AllowedDomainRepository)validationContext
                     .GetService(typeof(AllowedDomainRepository));

        string domain = value as string;

        if (repo.hasDomain(domain))
        {
            return ValidationResult.Success;
        }
        else 
        {
            return new ValidationResult(GetErrorMessage());
        }
    }
}

And I use it like this in my RegisterModel:

[BindProperty]
public InputModel Input { get; set; }

....

public class InputModel
{
    [Required]
    [EmailAddress]
    [AllowedDomain]
    [Display(Name = "Email")]
    public string Email { get; set; }
    ....
}

but somehow the [EmailAdress] attribute is working, as is the [Required] but I can't even get mine to fire (it's not reaching a breakpoint in the isValid method) even though Required and mine both inherit from ValidationAttribute.

EDIT: I got it to fire, once I post, but this is too late.

[EmailAdress] does fire while the input field looses focus which is what I want for my validation, too.