Mate,
because somehow elementor does not include the dependent libraries of the font awesome. there is some missing libraries on which these elements are dependent. if they are not included these won't be parsed as it should.
we can give you a code to dynamically add the dependent classes or the simplest way is to install the font awesome plugin. it will load all the required files automatically.
refer the link:
https://wordpress.org/plugins/font-awesome/
trust me this will work. try and let me know.