When the user attempts to add a Font Awesome thumbtack icon for pinned posts, the HTML element
<i class="fas fa-thumbtack"></i> used for the icon is automatically commented out in the HTML editor, preventing it from appearing on the page.
This behavior is caused by a conflict between the Font Awesome script and the redactor.
To resolve this issue, use the CSS instead of the JS file for the fonts. This way, the editor will not make any changes to your entries. Please replace the CDN from your header with the one here: https://fontawesome.com/get-started/web-fonts-with-css.
Credit: Manuel Gonzales