Update - 8/22/2021 12:21 PM CST
It looks like NVIDIA has released a fix that should mitigate the issues being seen with Android and Shield devices. They are currently monitoring the issue to see if anyone continues to have the problem.
Original Post -
According to NVIDIA's status page for GeForce Now, Fortnite is currently having issues with Android/Shield gamepads not working correctly.
They are currently investigating the issue, and we will update this article if any news from NVIDIA is posted.
Fortnite has Gamepad issues on Android and Shield
Investigating - There are some issues with Fortnite and Android/Shield Gamepads. We are currently looking into this. Thank you for your patience.