Please share URL where there are no icons! And you don't see them on all browsers, Chrome, Firefox, Edge etc.
But when a page generate from other components are showed, Font Awesome is not loaded and many symbols disappear.
In most cases it's caused when other component loads it's own font awesome file that is OLDER than v5.15.x then indeed becuase component files are loaded after template there could be conflict between Font Awesome versions. To be honest that mess was made by FA developer, he made a lots of changes in class names. You can fix it in few ways.
2nd reason. Becuase right now Font Awesome Free 5.15.1 is loaded only from SP Page Builder, not from Helix Template which was disabled by you, on some subpages where you do not have any SPPB object, template cannot use Font Awesome 5.15 becuase it's not loaded.