Sound bug when playing games?

I've been playing Fallout Shelter and Pokémon Go a lot lately and I noticed that if I have the sound on and I receive a notification, the in-game sound mutes. I'm not able to turn the sound back on without completely exiting and closing the app in the Recents screen. Does anyone else have this problem? It doesn't matter if it's a text, email, etc. Sound dies.

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