I'm having same problem on a Pixel 6a. I've flashed pixel 6a with GrapheneOS and PS4 Controller would connect from what I could see using bluetooth but uppon starting steam link or geforce now none of the buttons worked. Tried a few other apps just to test controller but none could detect any buttons pressed.
Flashed it back to stock and PS4 controller worked as expected both using steam link and geforce now. No problems at all.
Re-installed Graphene OS and same problem again, it connects but no input detected. I can see that it's connected as on a controller it self there is a light blinking when in pairing mode, but once paired the light stays lit.