This is a temperamental problem - most of the time everything is fine, but it occasionally comes up on a project.
I always use the physical camera with exposure gain set to manual. I don’t modify exposure settings in environment and effects. Sometimes the render in the frame buffer can jump to a darker or brighter setting - sometimes after it has completed, and sometimes when I’m re rendering a small area of the image. You can see the issue in the attached, when re-rendering the center region the rest of the image switched to a lower exposure.
