clear your system cache, go to the camera app settings and clear the data and cache. Also, recording in poor light will produce poor quality videos. Wipe down the camera lens as well.
It's your lack of lighting. Shoot the video with the window behind you and the quality will be much better. Anytime you take a photo or video with the main source of light in front of you the quality will be grainy.
Please try clearing the system cache and rebooting your device.
If issue persists, please send us an error report with screenshots from One UI Beta App (Beta Feedback -> Send feedback-> Error Reports -> "Select Category" -> Input frequency and Description" -> Send (Top Right Icon). We appreciate your help for supporting beta program.
