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Merge commit diff results are confusing and inconsistent

I'm hoping this is mostly a learning opportunity for me. I'm assuming
things are working as designed, but I just don't understand something

I have a merge commit. HEAD is currently pointing at this merge
commit. To be exact, HEAD points to master, which points to the merge
commit. My goal is to diff only the changes in the merge commit (stuff
committed directly in the merge commit, such as conflict resolutions).
To start out, I learned about @^@, @^!, and @^-. @^! sounded like what
I wanted. It gives me this output:

$ git rev-parse @^!

Perfect. This should give me just the diff of 21f5... and exclude
everything else, right? So I did this:

$ git diff @^!

However, I get *all* changes on the branch (second parent) and changes
in the merge commit itself. Basically it acts as if I used @^-, which
seems wrong to me. So to test another angle, I used the revisions
output by rev-parse directly:

$ git diff 21f5a4b9fee4f12e7793919f65361d2c16f7d240

Interestingly, this showed me only the changes in the merge commit
(21f5a4) and nothing else. Between this command and @^!, I feel the
two are exactly the same. So why does @^! not work as I expect, but
explicitly specifying the revisions does? What am I missing here?

When I use @^! in `git log`, I do only see the merge commit and no
other commits. So at least log is treating it correctly.

$ git version
git version 2.20.1.windows.1