Yes its the same on my phone as well.
Yes it is normal, No it is not working as it should.
Just because all the iphones have this problem, does not mean its working properly.
There is something wrong with the code that controls the sensor.
They need to fix this. But they probably wont because not enough people are complaining about it.
Its a light sensor, not a when i feel like it sensor.
Apple wanted the iPhone to control light this was since you can place you finger on the sensor while playing games or surfing the web or even sending a text message in landscape mode. Resulting with a dark screen which is hard to see in a bright area. So the iPhone only sets the brigtness upon unlock or moving to brighter areas on lock it so it won't disturb you.