Brightness changes whenever I change the app.

[AC Question]

I have Samsung galaxy tab 3v. It was working fine though , but suddenly I realized that the brightness of the tab restorer on its on . I usually set my brightness on zero (the least) bec I like it that way .. whenever i changeto another application, my brightness moves to 50% although I have no auto bright option on my tablet. I don't know what is happening .. Can someone answer my question plz. Thanks in advance