In order to prevent a more game-breaking issue, a change was made in the Zwift Mobile Link (ZML) app that restricts players from fan viewing anyone outside of their Riders Nearby list. The Riders Nearby list is based on the rider that is currently being viewed, whether that's your avatar or another player's.
While this change was made, the fan view button may still show up for a rider outside of this radius due to it updating in real-time, so you can press the button and still be unable to fan view that rider.
There's two options if you want to fan view other riders that are out of this radius:
In ZML, select the farthest rider you can and fan view them. The radius is based on the rider currently being viewed, so this will shift the radius, and you can then keep hopping further up the list.
You can also still fan view any rider you want in the Zwift game itself by choosing their name from the rider list on the right.