iPhone brightness gets set to the lowest after restart

Whenever I restart my iPhone, the brightness gets set to the lowest, so it is very dim. Then I have to adjust it back to around the middle, where I like it. How do I stop this from happening? I turn off my iPhone by holding the power button, then slide to turn off, then press and hold power button to turn on. I have auto-brightness disabled. I'm using an iPhone 6 with iOS 11.2.5.