Hi Noah,
on a project I ran into the same issue and we created a solution for Nuke at least. With Fusion I am not familiar.
As @ChrisBrejon explained, you need to think about “white” a bit different in ACES or with any 1D/3D-LUT workflow for that matter.
First you could render your scene in front of a which backdrop in the 3D scene. When enough light hits the wall, it will appear “white”. Think about filming your setup in a studio with a white backdrop. You need an enough even lit background that it will appear white in your camera.
While you are still working scene referred in your Fusion comp in ACEScg and use the view transform ACES/sRGB, you are already thinking that you are back in Photoshop where 255/255/255 in 8-bit is white. This can be only true after the view transform, because only then you can work again display referred.
The workaround in Nuke that we called “display-referred merge” takes you out of ACES and moves you back into it with a potential loss of dynamic range and an irreversible compression of gamut.
In Nuke you do the following steps:
ACEScg–>Out_sRGB
take off the sRGB gamma with a colorspace node
merge your 3D onto a “white” (1.0/1.0/1.0) background
apply again the sRGB gamma with a colorspace node
apply the inverse Out_sRGB —> ACEScg
Now you can view the merged result through the ACES pipeline. The white is now at 16.xxx, but your edges are fine. But remember you broke ACES chain by going scene referred–>display referred and back to scene referred again.
When this happens at the end of the comp it may be a good workaround.
I hope this helps
Daniel