My Galaxy S10 has just recently (last month or so) started to have this problem. I've had the phone for 2 years without any other issues. Just randomly the Notification and Systems sounds keep turning off. I'll turn them back on and they work for a while and then randomly again, they'll turn off. The other sounds arent affected at all. its just annoying.. I cant help but think that some signal was sent to my device since its paid off... lol.
I've reviewed the different message boards and nothing seems to fix my exact problem.
I've had the apparent same issues from time to time, but it's always been the result of something in my settings, like the focus feature, or the bedtime features. You're just going to have to go and poke through your settings to find the culprit or reset the phone to the factory settings and start over with the phone.