I need some help understanding an exception I get when I use a surface controller using both a child and post action.

I have an account controller with a child action and a post action

public class AccountController : SurfaceController
{
    public UmbracoHelper umbracoHelper => new UmbracoHelper(UmbracoContext.Current);
    private IPublishedContent AccountPage => Umbraco.TypedContentAtRoot().FirstOrDefault(x => x.DocumentTypeAlias == "memberControls").Children.FirstOrDefault(x => x.DocumentTypeAlias == "account");

    public ActionResult Login()
    {
        return PartialView(new CustomLoginModel());
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Login(CustomLoginModel model)
    {
        if (!ModelState.IsValid) return CurrentUmbracoPage();

        if (!Members.Login(model.Email, model.Password))
        {
            ModelState.AddModelError("loginModel", "The email or password is invalid");
            return CurrentUmbracoPage();
        }

        return RedirectToUmbracoPage(AccountPage.Id);
    }
}

From my login page, I'm including my form as a partial view by calling the action

@Html.Action("Login", "Account")

Now, if I submit my form correctly and I redirect to the current umbraco page or another page everything works as expected. It's when the form has an error and I return the current umbraco page that I get the error mentioned above.

I have found that if I include the child action and the post action in different surface controllers that I can return the current umbraco page without getting the above error. While this seems to work, I'd really rather find a way to keep my logic in the same controller. I don't get why separating the child and post actions behaves as I would expect while combining them in the same controller would result in an error.

Any insights would be appreciated. Seems like this may have been touched on briefly here: https://our.umbraco.com/forum/developers/api-questions/36614-411-Using-SurfaceController-Child-Action-with-Post#comment-132482