If a page is loaded with the iframe embed code and the viewport changes (browser window resized, orientation change, etc.) before the embed element is scrolled into view, upon scrolling the element into view the contents are not properly sized to fill the frame. This is not an issue with other iframe content (including similar embedded storefront widgets like Humble, Steam, etc).
I haven't verified this but it also seems like the embed is lazy-loading itself which is arguably against best-practices in this case, as it would be best left to the parent page to optionally implement.
I've solved this issue on my end for now by setting and clearing the width of the embed element when it enters view upon resize or scroll, but the root problem should be addressed.