I usually have my phone on ringer so I'm not sure how long I've been having this issue. I noticed it this morning because I wanted to stop getting sound notifications for a game I was playing. After playing the game for a few minutes, I go back to my homescreen and the ringer is on again. I downloaded a ringer/vibrate/DND toggle to see if that would help, but it hasn't. It does the same thing if I put it in DND mode. After a few minutes or after the screen shuts off, it always comes back on.