Did you already force restart your iPhone after the update?
Force restart iPhone - Apple Support
If this does not work, set it back to factory settings, without using the latest backup and test the camera again:
Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support
Восстановление заводских настроек iPhone, iPad или iPod с помощью компьютера - Служба поддержки Apple (RU)
Also make sure that nothing is covering the space around the camera lens, for example a case of magnetic clip.
My iPhone 12mini camera is working without any problems after the iOS 18.1 update.