A small point to add onto @awesomerobot’s comments,
FontAwesome recently updated their icon set to version 6.0.0 and as a result there were new icons added and some icons had their names slightly changed.
I believe Discourse is still using version 5.15.4, which is why you may be seeing the issue as the user-group icon does not exist in that version. Perhaps you might want to use user-friends as its a similar icon.
You can filter by v5 by using this link: