Facing exactly same behaviour. In iOS 13 and above silent push is received even app is forcefully quit but not in earlier version in iOS 12/11. In iOS 12/11 silent push only received when not terminated by user (background/foreground).
I had asked same question on stackoverflow - https://stackoverflow.com/questions/58128321/why-my-app-in-ios-13-awake-in-background-by-silent-push-when-terminated-by-user as well.