A likely reason you are not able to see the change in the preview is that your coming-soon.php template doesn’t include the requisite wp_head() an wp_footer() calls being made. If you look at your browser’s DOM, you should actually see that a secondary iframe is getting created behind the visible iframe. However, if your template doesn’t include the standard theme hooks then the customize-preview.js script won’t be enqueued and it won’t be able to communicate to the parent window that the iframe has loaded and thus is ready to replace the previous iframe.
This same question was actually reported to WP Core Trac in #37981: “template_include not working in customizer previewer” in which I answered similarly:
you have to make sure that the
wp_head()andwp_footer()calls are made so that scripts are printed. Ifcustomize-preview.jsisn’t output, then the loaded page won’t be able to communicate to the parent frame that the page has finished loading. This issue actually came up recently elsewhere: https://github.com/Automattic/amp-wp/issues/477#issuecomment-247679386