`checked` attribute on radio button input does not exist

When trying to create an radio button input and setting the checked attribute, the checked attribute never appears. Interestingly, the CSS for :checked state elements works as does VoiceOver read out for the selected radio button at the time.

Here's an Ember twiddle that displays this issue:

https://ember-twiddle.com/1c3c6686c49a0c8338e24acc66b05a26

You'll see that the checked attribute never appears on the selected radio button, but both the DOM element and CSS display as checked or selected.

Does anyone know what might be causing this apparent conflict?