If <template> is rendered by glimmer, its content (DocumentFragment) results to be empty.
Tried on ember 3.18 (ember-twiddle), 3.28 (local), and try.glimmerjs.com.

Reproduction:

1. Go to demo on https://try.glimmerjs.com/
2. Inspect document.getElementById('a').content.children.
   Expected: HTMLCollection(2) [div, div]
   Actual: empty HTMLCollection []

I could be wrong, but it seems to me that glimmer renders <template> like a normal HTMLElement, doing templateElement.appendChild instead of templateElement.content.appendChild, or so.

Chrome:

Firefox:

Failing test: #370.