Looking at the oEmbed response from ellie:
{
"height": "height",
"html": "<iframe src=\"https://ellie-app.com/embed/6dpbWF9xra1/0\" width=800 height=400 frameBorder=\"0\" allowtransparency=\"true\"></iframe>",
"provider_name": "ellie-app.com",
"provider_url": "https://ellie-app.com",
"title": "",
"type": "rich",
"version": "1.0",
"width": "width"
}
and a quick skim of the oEmbed spec, it looks like the width and height params are required and should be the pixel dimensions of the result. Perhaps that has something to do with it?