All I can suggest it raising an Error Report in the Members app.
I have the same problem with system sounds (sometimes if you go to adjust the volume in a system app then it's like "hell, what on Earth?")
I've disabled evil FB and most active notifications (with sounds - this seems to work). So I can't comment on app specific stuff.
Mine is now working fine. It's using the system sounds volume and following it correctly. I think it's something facebook fixed since that's had a few new versions and none from Samsung.
I know this is bumping a slightly old topic, but this is not fixed for me. Facebook, Messenger, Google Messages (SMS app), Office Lens, loads of others still have incredibly loud volumes for In App sounds despite all volume bars being at the lowest.
Clearly, the fault is with the OS (whether the Google side or Samsung side, or both!) rather than the apps.