FYI, the webpage embedded in the dashboard is not loaded via the server, it's loaded locally on the client PC.
This is almost certainly going to be a problem with mixed HTTP / HTTPS connections. If you access your Tableau Server using HTTPS but you are using an HTTP URL in your webpage part, you'll have problems loading mixed content and the inline page may fail to load.
Try alternating the URL you use to access your Tableau Server between HTTP and HTTPS and see if that fixes the problem. You can also try alternating the URL used for the web page part between HTTP and HTTPS as well.
Also, this is specific to Google (and other sites which impose the same restriction):
When you embed a webpage in a Tableau Dashboard and render it on the web, it uses a HTML object called an iframe. Some webpages / servers specifically block their content from being embedded in iframes.
If you look in the console / developer tools for your browser, you'll see if any errors are being generated. Sure enough, embedding Google shows;
"Refused to display 'https://www.google.com/' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'."
You can google the error if you'd like to learn more but effectively, Google doesn't allow it.