I have a new (October 2020) S20 5g. When I first start the phone, the volume on notifications is fine. After an hour or so, the volume is automatically reduced about 80%. This does not seem to happon on the ringer, only on notifications. Bixby has been deactivated and all volume levels including the system level ar fully up. The do not disburb toggle is off. If I restart the phone, the volume goes back to up and in an hour is is reduced again. Samsung tech took over my phone and checked all the settings. Cannot fix this but have seen numerous similar complaints on the web.
Could it be the recent Android update? However, complaints go back a couple of years.