Question marked as
Any chance your display has been replaced in the past or is it a refurbished iPhone?
If not, and to rule out a software issue, force restart your iPhone:
Force restart iPhone - Apple Support
In case this does not bring back the controls for Auto Brightness, restoring your phone to factory settings on your computer would be the next step:
Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support
If still no luck, contact the official Apple Support, or visit an Authorised Apple Service Provider next to your place:
Contact Apple for support and service - Apple Support