If your GoPro Hero 9's auto-rotate feature has stopped working, there are a few things you can try to fix the issue: Check if the Auto Rotate feature is turned on, Go to Settings > System > About > Camera Info, and check if the firmware is up to date.
If none of these steps resolve the issue, it's possible that there is a hardware problem with the camera. In this case, you should contact GoPro support for further assistance.