Just did a factory reset, and in setting up, I've chosen my custom ringtone (got it on my Galaxy S3, and carried it to every new phone since), and a stock Note 10+ notification sound. This is standard stuff; done for every phone since my original Galaxy S Fascinate. (To be clear, I'm not asking about setting a custom ringtone or notification sound.)
Someone posted about the same problem in November, but just dropped the thread without resolution. Spontaneously, and at random intervals, my phone is changing my ringtone and notification sound to something different. It's always set to the same thing, and it's a selection I've never entered. Oddly, the default notification sound it chooses is actually a ringtone. So when I change back to the notification sound I want, I don't even see as an option the sound my phone keeps coming back to.
Oddly, paired with those two sound is the vibration pattern. I can change it, and my change persists. Also, I got a notification yesterday just 3 minutes after the time for do not disturb mode to kick in. I had the screen on, and had just changed the sound back to my choices when I got a notification. It sounded with the sound I don't want, AND the sound kept repeating (it's a ringtone, but doesn't normally repeat), despite the fact that no notifications should sound at all during DND.
Does anyone know what might cause these sound to change? Is there a system log I can check?