The ArcGIS Online April 2018 update just introduced the Media Map, "a simple UI that is suited for embedding in web media." This is great, because I'm working on creating maps for our new website and I love how simple and customizable this template is (hiding zoom buttons, etc.). However, I've put Arcade into my popups (feature name will result in returning the correct website string), and they aren't displaying on the final application. See the images below.
Embedded App or View Application from ArcGIS Online:
ArcGIS Online Map Viewer of underlying web map: