After replacing the display (full assembly) on a 5s, I had a situation where the toggle was gone and there was no auto adjustment happening when I changed lighting conditions. I determined this was a hardware failure (specifically the ambient light sensor) and got a new display assembly (including the ambient light sensor). Then, the toggle was still missing, but it was actually auto adjusting the brightness, showing that it actually was working. To fix this, I did a factory reset and restored (from iCloud backup in my case). The toggle came back, and all with that was fine.