auto brightness

Have you checked Settings > General > Accessibility > Display Accommodations?

Adjust the brightness on your iPhone, iPad, or iPod touch - Apple Support

Look around at your Settings. Perhaps, for example, you've set Auto-Brightness under ... Accessibility > Magnifier.

We would need a highly detailed explanation of your scenario to answer a "why" question.

I'm having the same issue as well, auto brightness doesn't appear to be working. Not sure if it's related to the new IOS update or not, but I noticed that yesterday it hasn't been working.

EDIT: The update broke it. My wife wasn't running 11.3.1 and auto-brightness was working. Not working after the update.

In my iphone that option is missing. Why??

You were right... the auto bright was the, but it seems not to be working as it use to be.

I went to the repair and for all that they told me, they made a software restore and now it works fine