iPhone上的HTML5视频标签并不总是在iframe中运行

We have a page with an iframe, inside which we have an HTML5 video tag. On the iPhone, we're having a problem where it seems the video tag is not always recognized. When this happens, it doesn't get the poster frame or render a play button when it's ready. This is never a problem when the page is reloaded, but always happens when the page is visited by pressing the URL bar and pressing "Go" or by visiting the page from another page on the site.We have a page with an iframe, inside which we