The Sticky Sidebar settings in your theme is causing this issue.
Site Reviews loads the reCAPTCHA widget in isolated mode so as not to interfere with existing reCAPTCHA installations on a page. This means that reCAPTCHA inserts the widget with the Review Form element instead of in the root of the page.
reCAPTCHA uses position:fixed; to position the widget relative to the viewport of the page instead of to the relative position of the surrounding elements in the DOM.
The problem with your theme’s Sticky Sidebar implementation is that it uses a combination of position:fixed; and transform:translateY(); to make the sidebar elements sticky.
Normally nested position:fixed; elements are positioned relative to the viewport of the window. However, when combined with transform, a nested fixed element will use the parent fixed element as the containing block instead of the viewport. This is what is happening on your page.
See also: https://developer.mozilla.org/en-US/docs/Web/CSS/position