I upgraded to Android 9 and my FM radio doesn't work either. I did discover, however, if you press and hold the headset Play/Pause button for several seconds while the radio app is open, audio will return. You may have to do this twice.
The problem seems to be audio related because the FM tuner will scan and locate stations, but with no sound. Holding the headset Play/Pause button for a few seconds solved the problem for me. That said, it's just a work-around for the problem.
I've tried this with the ASUS-provided headset and another non-ASUS headset and this trick worked for both. Hopefully a future software or OS update will fix this.
See if that helps.