Changing the opacity of this black video underneath my main video footage changes the brightness of my main video:

https://drive.google.com/file/d/1OEuBbvjEr1cEkpMNqY0TMyzBOQjOQ08e/view

Turning off mercury playback / mercury transmit fixes it. But I obviously don't want to edit Software only rendering as it's a massive performance hit.