I have an AGOL Instant App (Zone Lookup) embedded on a webpage using the embed code provided by the Instant App config, e.g.:
<iframe src="https://[myOrg].maps.arcgis.com/apps/instant/lookup/index.html?appid=[myappID]" width="400" height="600" frameborder="0" style="border:0" allowfullscreen>iFrames are not supported on this page.</iframe>
However, there is no option in the app to make it fullscreen. See screenshot below.

Is this a known bug?
Regards,
Jason