Can't reference custom widget inside UI file

Problem: I've defined a widget MyBin but using it inside a UI file brings up an error.
JS code

import GObject from "gi://GObject";
import Gtk from "gi://Gtk?version=4.0";
import Adw from "gi://Adw";

Gtk.init();

const MyBin = GObject.registerClass(
  {
    GTypeName: "MyBin",
  },
  class MyBin extends Adw.Bin {},
);

const MyBox = GObject.registerClass(
  {
    GTypeName: "MyBox",
    Template: workbench.template,
  },
  class MyBox extends Gtk.Box {},
);

const b = new MyBox();
workbench.preview(b);

blueprint (but the problem seems to be present even with normal XML. You can translate between the two...).

using Gtk 4.0;

template MyBox : Gtk.Box {
  .MyBin {}
}

Error:
(re.sonny.Workbench:4): re.sonny.Workbench-WARNING **: 22:51:26.278: Requiring My, version none: Typelib file for namespace 'My' (any version) not found

Notes:
console.log(MyBin.$gtype.name) outputs MyBin1, MyBin2, ..., MyBin${i}, incrementing every time I click run. The name of the type seems to be changing. Even knowing that, setting in the UI file the type MyBin3 and trying to hit run until MyBin.$gtype.name == 'MyBin3' doesn't fix anything.

Start with the JavaScript and Blueprint reproducer in the issue, then compare the Blueprint behavior with the reported XML behavior. Re-run the example and inspect the namespace warning and changing MyBin type names; done means the custom widget can be referenced from the UI file without the warning.