iPhone Brightness

The brightness on my iPhone keeps growing darker then brighter and I can’t figure out why. It’s not changing like normal in a smooth motion such as automatic brightness tends to do, it changes like switching off a light. It’s nice and bright then BAM someone turned off the light inside my iPhone and it grew darker. Yet, the brightness scroller in the control center stays on high (100%) the whole time. And then after a while someone turns the lights back on inside my iPhone and I can see again. And it always changes like a flash. Never the nice fade automatic brightness has. Any suggestions in resolving this issue will be greatly appreciated.