Since the December 1st Android update was installed I've experienced two noticeable issues on my S9:
1) Battery life is considerably lower. There's been no change in app\phone usage apart from the Dec 1 update
2) The People content in my Edge panel keep disappearing. I have to remove\re-add the People option for the content to reappear
3) Notification sounds are heard on a random basis - sometimes they ring, sometimes they don't
As mentioned there's been no change in the phone's apps\usage and the issues above have only arisen since the Dec 1 update.
Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way.
Also please put the phone in Safe Mode which disables your downloaded app's but let's your stock app's keep working. If the issues stop then this highlights a downloaded app as the possible issue. Come out of Safe Mode and start deleting your most recent downloaded app's one at a time until it stops.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.