Sadly that is the nature of iframes, so our objects will scale to the available size when the page was first rendered.
There are workarounds, we make use of pym.js (Pym.js: Embed iframes responsively) from NPR heavily for our deployments. So the parent page will notify the iframe when it's height and width has changed and then our resize handlers should fire accordingly.
Is the problem really has anything to do with iframe? Could you explain a bit please? I'm not sure I understand unfortunately. If I host the regular app (not single) in the same iframe, everything renders responsively.
So, when I resize iframe, its content layout is changed correctly which is not the case for single app.
http://localhost:4848/single/?appid=<myappid>&sheet=Yqwnms&opt=nointeraction&select=clearall- renders incorrectly with iframe resize. Content visualizations are scaled and layout stays the same
http://localhost:4848/sense/app/<myappid>/sheet/Yqwnms/state/analysis - renders correctly with iframe resize and layout is updated fine but contains toolbar allowing user to explore hub or edit sheets.