IBDesignable's only render in Storyboard, not in standalone XIB's

I haven't read anywhere yet if this is normal, but I can't get IBDesignable views to render in a standalone xib, such as a custom view or even a TableViewCell.

When I put them in a storyboard, they render perfectly fine. Here's an example for reference of my IBDesignable custom view:

@IBDesignable
class AvatarView: UIView, NibLoadable {

    @IBOutlet public var contentView: UIView!
    @IBOutlet public var backgroundView: CircleView!
    @IBOutlet public var label: UILabel!
    @IBOutlet public var imageView: UIImageView!

    @IBInspectable var labelText: String = "" {
        didSet {
            self.label.text = labelText
        }
    }

    @IBInspectable var image: UIImage? = nil {
        didSet {
            self.imageView.image = image
        }
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        setupFromNib()
        commonInit()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupFromNib()
        commonInit()
    }

    private func commonInit() {
        clipsToBounds = true
        layer.masksToBounds = true
        self.backgroundColor = .clear
    }

    public func setUser(_ user: User) {
        labelText = getInitialsForUser(user)
    }

    public func setImageUrl(_ urlString: String) {
        self.imageView.kf.setImage(with: URL(string: urlString)!)
    }


    //MARK -- PRIVATE --

    fileprivate func getInitialsForUser(_ user: User) -> String {
        var name = ""
        if let fullName = user.getFullName() {
            name = fullName
        }
        return name.components(separatedBy: " ").reduce("") { ($0 == "" ? "" : "\($0.first!)") + "\($1.first!)" }
    }

}

Here is the NibLoadable class as well:

public protocol NibLoadable {
    static var nibName: String { get }
}

public extension NibLoadable where Self: UIView {

    static var nibName: String {
        return String(describing: Self.self) // defaults to the name of the class implementing this protocol.
    }

    static var nib: UINib {
        let bundle = Bundle(for: Self.self)
        return UINib(nibName: Self.nibName, bundle: bundle)
    }

    func setupFromNib() {
        guard let view = Self.nib.instantiate(withOwner: self, options: nil).first as? UIView else { fatalError("Error loading \(self) from nib") }
        addSubview(view)
        view.translatesAutoresizingMaskIntoConstraints = false
        view.leadingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.leadingAnchor, constant: 0).isActive = true
        view.topAnchor.constraint(equalTo: self.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true
        view.trailingAnchor.constraint(equalTo: self.safeAreaLayoutGuide.trailingAnchor, constant: 0).isActive = true
        view.bottomAnchor.constraint(equalTo: self.safeAreaLayoutGuide.bottomAnchor, constant: 0).isActive = true
    }
}

One can use @IBDesignable views inside a NIB/XIB. E.g.

One simply cannot access any project resources (NIBs, custom named colors, assets, etc.) programmatically from within an @IBDesignable view. Designables rendered in Interface Builder do not have access to your bundle. Designables run in a stand-alone environment.

So, yes, you can use designables in NIBs, but just not the other way around.