Hi there and thanks for all the details!
iPhone's iOS still doesn't have native full-screen support... If you were not
using iFrames, the viewer would still show the full-screen button on iOS and it would open your 360 product view in the "full browser window" mode (i.e would take the entire browser window).
But since you are using iFrame, there's no way for the viewer to spring out of the iFrame controlled by your page and expand in the full browser window mode and so we auto-hide the button when inside an iFrame on iOS.