@cyber: That's strange. Please can you try heading to Settings > Display > Adaptive Brightness, and toggle this option off. Similarly, scroll down to Screen Timeout, and set this to a higher option such as 5 minutes or 10 minutes. If the problem persists, please head to Settings > Apps > Camera > Force Stop > Storage > Clear cache, then restart the device. Let me know if this helps.
@cyber: I understand that it isn't convenient, however it's important to explore any potential causes of the screen dimming at this stage. You could add a Bixby Routine to increase the brightness when you open the Camera app as a workaround by heading to Settings > Advanced Features > Bixby Routines > Add Routine > If > App Opened > Camera > Then > Display > Brightness > Move the slider to the right to your preference.