Why iphone screen goes dim itself when i disconnect call?

It only happens when i make a call.
i've noticed auto brightness is off, screen brightness is set at 50 percent, let i disconnect call it goes to zero brightness
if i scroll setting & brightness its still at 50 percent but screen is still dim, if i drag brightness icon it goes normal,
please tell why its happening?

Try to open Automatic backlight to check it.

The screen will automatically dim ~10 seconds before the screen goes to screen lock mode. The time is determined under what is set at Settings>General>Auto-lock

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