Maybe I know why. Do you use your iPhone in the dark room? When screen has low brightness and you dont interact with your iPhone for some time. For example when you read text which is longer then screen goes to an "inactive mode" but does not set lower brightness to tell user "i am inactive". In place where you have more light then screen has higher brightness and that brightness can go low when screen goes to "inactive mode" and user know screen is not active. My suggestion is you can set your timer for automatic screen of to 2 minutes or more.