Even before coding harnesses become mainstream, a lot of tools offered to automatically generate commit messages based on the diff (I think JetBrains IDEs started to offer it very early) and I always cringed when I've seen it.
The reason why I don't like diff-base commit messages is because they are redundant. If I want an LLM-generated summary of the diff, I can easily generate it myself, there is absolutely no reason to put it in the commit message.
What I would like to see in the commit message is some additional context that is not a part of the diff. I don't need to read what has changed, because I can already can see it in the diff (or get an LLM to summarize it for me). But I often do need to understand why this change was made. What were they trying to achieve? That's the important part that is not contained in the diff itself. And this is the part that diff-base commits rarely contain.
I think that even something simple like a link to a Jira (or other bugtracker ticket) is much more helpful than diff summary. Maybe instead of "fix" or "update" you could write just a couple of words about what are you trying to fix and why does it need updated. Still better, than a diff-summary.