Thank you very much, battle kid. The file is a great help!
I have rendered the scene, then saved it from the Picture Viewer as a PSD and a TGA. Open both in Photoshop, and I can't see a difference. I even switched the Color engine in PS back to sRGB to see if that helps (I typically stay in larger color spaces. However: No difference.
Getting closer (but not matching your results) if the Image is assigned (Not converted to!) a Color Profile of my Monitor "DCP-P3" instead of the small sRGB. In that moment, the comparison comes very close to your comparison. Since that has no indicators and is only close to your observation, I exclude this here.
What I can tell, if you got the PSD from the Multipass (which should be in float to work properly for all), you get a Linear encoded pixel value in the file. But that is not how the file will be read. When compared then with 8bit/channel float, while ACES was used, a list of changes are created, to avoid clipping of dynamic and gamut, which changes values. (Working against those changes, is not advised, as in 3D we work with scenes, not just images. Which is a longer story, and not the concern here.)
Since Multipass isn't working very well with formats that typically work with Gamma-based pixel value encoding, any integer formats are excluded.
In comparison, I get what you experience.
I did set up a "gray-value" test to take the guesswork out.
To be certain that I did not miss any settings in your file, I copied the content into a new project and rendered it as well. The files are also in the folder
https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20251219_CV4_2026_drs_25_RSct_01.zip
In short, images without context are guesswork, and not what I like to do. However, what I can't do is to simulate your workflow context, hence the images in the link.
TGA and PNG often have no color space profile added, but their pixel values are, of course, encoded, as their main use is in sRGB. Which is a delivery format for web, for example, not a format that should be used in 8-bit/channel quality inside the production pipeline. Yes, they are smaller, and yet that often leads to problems. If used in Linear, sRGB has a very limited Gamut, which leads to other problems. But that is a different theme.
What I need to know are the files matching in Photoshop, except the Multipass files.
Please let me know, as well, what color engine you have set up in Photoshop. Or if that is nothing that was changed.
Fingers crossed, we found the problem.
I have no idea about your background in Color Management, so I might suggest something that you know already. Since I write in a forum, I always share a little bit more.
If you like to dive deeper, I can suggest one or two books, as Color Management is simple when the underlying knowledge is given, but it takes some time; there is no shortcut. I suggest books here, as professional courses are either expensive or long, and many sources on YouTube cut corners and often miss the point.
Cheers