A quick update, everyone. We seem to have fixed it, but I believe this is a bug with Unreal Engine.
As you can see, if the Lumen final gathering quality in your Post Processing is set to 0.5, the scene becomes really dark and renders incorrectly.
However, changing this value to a higher / lower one will completely fix the issue. As you can see here, I have changed the final gather quality to 0.25 and then to 1.
Other values will work, but there seems to be a threshold where everything darkens. Something between 0.4 and 0.8. As you can see in this short video, the behaviour is really weird. It almost appears as if something is switching on and off. Notice how little I’m tweaking the value.
This might explain why the issue doesn’t seem to be wide spread. I assume most people aren’t interested in tinkering with the lumen gathering settings. If your lighting looks different, perhaps it’s because your values are close to 0.5
Hope this can help someone