Many users face this problem because if phone comes under direct harsh sunlight, phone generates heat and cpu starts to throttle to cool down phone temperature and therefore cpu decreases phone brightness to help manage temperature but this is common problem in all brand phone.
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As per your issue, we request you to please report the problems with a log file. To share a log file, click on the Samsung Members Application (Open Samsung Members Application > Support> Tap on error report > Type your query > Send).
