As I originally responded I was seeing no difference in the blends when I checked for versions 4.10-4.12 Preview 5. Looking at the 4.10, 4.11, and final 4.12 in the content examples for the post process blend example (stand 1.15). I checked these again to make sure that there haven’t been any changes and I see no noticeable difference between these two blendables.
If you’re having a specific issue that is a difference in how the post process volumes worked from a previous version to 4.12 feel free to enter a separate ticket for this to be investigated. Make sure to include the necessary information to reproduce the issue or provide a sample project that reproduces the differences between a previous version and the latest release version.