I've seen a few other posts about this, but I was having issues with the media volume jumping around on its own after upgrading to One UI 5 and I tried all the other solutions I'd seen short of doing a factory reset (clear cache partition, uninstall Sound Assistant, repair apps).
I noticed after restarting a few times that the issue would only happen after using Facebook or Instagram. I uninstalled both apps and reinstalled them, and it seems okay so far.
Hope this might help someone else with the same issue!
