Hi there,
This could happen if your site has icon markup that you added directly rather than using our icon shortcode. As of Font Awesome version 5 they split icons into multiple fonts. This can be fixed by changing instances of the data-x-icon attribute to one of the following:
data-x-icon-b for social icons.
data-x-icon-o for outline icons.
data-x-icon-s for solid icons.
The original attribute data-x-icon will still work for solid icons as a fallback but we’ve updated everything for consistency.
Please update the file you have added to the functions.php and change data-x-icon to data-x-icon-b.
Hope this helps.