I have a web experience app created via ArcGIS.com (not developer). When I add a button widget and publish the app the button widget is not visible when the app is embedded in an iFrame. Is there an attribute that needs to be set in the iFrame HTML element? I notice also that the allow="fullscreen" attribute does not work with the full-screen widget when the app is embedded in an iFrame.
I would double-check the settings on the button. How is its position defined? Make sure that any anchoring / spacing is capable of adjusting for a variable layout. It's possible the button's there, but has so much spacing on one of its sides that it is being pushed out of the frame.
Can you link to the experience if it's shared publicly?