Thanks Dave and Steve for responding. I tried the methods you describe – spent time playing with contrast and brightness, used other additive modes (lighten, screen, burn) and non of them behaved correctly. I also, added a Levels effect, and precomped. I changed the bit depth from 8 to 32 and back again.
Only rendering out the layer and reimporting worked for me.
For now I’m just assuming there’s something exotic about Fractal Noise.