I'm also having the exact same issue where Facebook and FB messenger sounds are very loud. Lowering system and notification volumes doesn't change anything. I just uninstalled and re-installed Facebook and same issue.
I did 2 more things before reinstalling the app. 1. Wiped system cache from bootloader 2. Tried the "Repair App" from the bootloader menu who bring the optimizing after an update the next reboot. After these 2 things the volume was still high, and reinstall did the trick. Hope this help.
Got issue with facebook app too, unable to correct this. i.e. when clicking "Like" if volume is at 1% or at 100% it always sounds at 100%, same thing with my Galaxy Tab S7+. My wife Galaxy Tab S5e and Galaxy S9 don't have this problem so i suspect an update on recent Samsung devices.