Assets that are only referenced in Rich Text Field not appearing in asset collection

[mlawd]

I have a custom Post type that has two elements: and Asset element and a Rich Text element.
I have 2 images uploaded to assets.
1 has been selected for the Asset element and this appears in the assets collection.
The other asset is embedded in the rich text field. This asset does not appear in the assets collection.

The asset element in the rich text html passes in the URL as the id so I can render the images, however the fact that the full asset is not in the in the asset collection means I cannot add the asset description to the alt tag or request different sizes/formats via the api.

I believe I have a potential fix which I'm happy to push to a branch but wanted to check I haven't missed some configuration or done something wrong