embedding an app into the Hub Site does not allow fullscreen on the iframe element.
This leads to fullscreen is not working in an embedded apps.
Please, add the allow="fullscreen" attribute to the iframe element used by embedded content.
@MatejVrtich thanks for your suggestion, please post this on the ArcGIS Hub Ideas Board to allow your fellow community members to upvote your suggestion. This will raise awareness to the product team for prioritization.