Context: This occurs in crossplay between Steam (PC) and Playstation 5 (physical console). There's currently a bug where I cannot see my friends if they are online and in the same server as I am. ...
Ideas
It was happening to me between gullcrest village and market mile. It started with invisible friend, then the rest of people I saw. I finished the Purple Market mile mission and I started to see others again. Try finishing the Purple Market Mile mission.