It looks like you’re still using Font Awesome 4 on most pages, but Font Awesome 5 is loading on those other pages from your accordion plugin.
Using Font Awesome 5 is the right way, so you’ll want to:
1. Deactivate Font Awesome in Customize > General.
2. Use a plugin like this to add Font Awesome to your site: https://wordpress.org/plugins/font-awesome/
Then you’ll want to update your icon in your menu to use the Font Awesome 5 class names.
Let me know if this helps or not 🙂