Yes, this is normal. The ambient light sensor only activates when the phone is unlocked. It has happened to all my iPhones, (I have owned a total of 7 because I like to drop them ) it can be quite annoying to be honest.
This always annoyed me. When lying in bed I'll go to reach for the phone and of course, wake it up. Cue me being blinded by the light only to slide away into a more netural brightness, or course by then it's too late.
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The reason for this is simple, many people turn on the lock screen before the phone is in the right position to use. So the ambient light sensor doesn't get an accurate reading. Ideally when you unlock it, that's the right time to get a reading.
Not the ambient light sensor for me but when I turn on auto brightness the home screen dims down when unlocked. If turned off the lock and home screen have the same amount of light emitting from my phones LCD.