Required data annotations returning namespace rather than error message value?

I have an ASP.NET Core 8 MVC application, using the bog standard [Required] attribute on a class property. I expected to see the defined error message in my HTML, in the data-val-required attribute on the input.

But it's weirdly showing the namespace instead?

Here is my class:

namespace Test.Features.Authentication.ViewModels.LoginPageViewModel;

public class LoginPageViewModel : IPageViewModel<LoginPage>
{
    public LoginPageViewModel(){}

    public LoginPageViewModel(LoginPage currentPage)
    {
        CurrentPage = currentPage;
    }

    public LoginPage? CurrentPage { get; set; }

    [Required(ErrorMessage = "Required Message Here")]
    [Display(Name = "Email Address")]
    public string? EmailAddress { get; set; }

    [Required(ErrorMessage = "Required Message Here")]
    [Display(Name = "Password")]
    public string? Password { get; set; }
}

This is the Razor view:

<form class="form-inline" asp-controller="LoginPage" asp-action="Index" method="post">

    @Html.AntiForgeryToken()

    <div>
        <label asp-for="EmailAddress">@Model.CurrentPage.EmailAddressLabel</label>
        <input asp-for="EmailAddress">
        <span asp-validation-for="EmailAddress"></span>
    </div>

    <div>
        <label asp-for="Password">@Model.CurrentPage.PasswordLabel</label>
        <input asp-for="Password" type="password" />
        <span asp-validation-for="Password"></span>
    </div>

    <button type="submit">
        Login
    </button>
</form>

And this is the rendered HTML:

<input type="text" data-val="true" data-val-required="Test.Features.Authentication.ViewModels.LoginPageViewModel.EmailAddress-Required" id="EmailAddress" name="EmailAddress" value="" class="input-validation-error" aria-describedby="EmailAddress-error" aria-invalid="true">

I've never encountered this before... Any help is appreciated