I think this may be wrong, though apologies in advance if not…
For example, to use the address card icon at Address Card Icon | Font Awesome you need to change far fa-address-card to far-address-card (and add that to the svg icon subset setting). Simply using it without the fa- does not work.
I only realised this after looking at FEATURE: Update Font Awesome to v5.4.1 and SVGs by pmusaraj · Pull Request #6557 · discourse/discourse · GitHub and FEATURE: Update Font Awesome to v5.4.1 and SVGs by pmusaraj · Pull Request #6557 · discourse/discourse · GitHub