Why is my phone automatically setting to sound after I turn it to mute and turn it off?

Whenever I set my phone to mute, and turn it off, it goes back to sound. Thus has happened ever since I have installed a third-party theme and activated it through Samsung themes. I use a Galaxy A7 2017 (SM-A720S). Pleases help me, it's so annoying when I'm trying to sleep.

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