My best guess is something to do with how I'm somewhat abusing Partial to allow my service to accept the form data, since I didn't want my controller directly creating a wishlist, but rather my service.
const wishlist: Partial<Wishlist> = {
name: this.wishlistForm.value.name!, // I use ! to disallow `null` typing, which Partial doesn't like
description: this.wishlistForm.value.description!, // ^
};