iOS 14.5 auto brightness button disappeared

Like I said, the screen your iPhone is using is either not a legitimate or has failed the calibration. Go back to place where you did replace the screen to have them to fix it for you or to get your money back.

If you’re referring to the “Automatic” button under Settings > Display & Brightness, and you’re saying that switch no longer appears, then perhaps either Low Power Mode or an Accessibility setting is preventing its appearance.

Did you replaced the screen on your iPhone? And if yes, was it done by Apple or one of their Authorised Service Providers? If not, then possibly the screen you use now isn’t a legitimate one or wasn’t properly calibrated.

Where are you looking for this option on your iPhone? It should be under Settings > Accessibility > Display & Text Size.