Magit rebase: empty magit-diff while rewording commits

I’m trying to get into using magit instead of git on the command line. I really like how magit-diff pops open when commiting staged changes, as it makes it much easier to record all the minutiae in the commit message and not leave important bits out by forgetting the details.

Now I have a series of commits where (unfortunately) I didn’t take the care I usually do while writing out the commit messages. I was hoping that rewording commits in magit would allow me to see the diff for each change (as when committing), but it seems magit-diff doesn’t display anything useful, only an empty “Staged changes” view (see screenshot below).

I wonder if anyone else has run into this and if they know how to do what I want – see the diff for the commit currently being rebased/reworded – or if it’s just not possible. I’ve been googling for a while, speed watching some of @daviwil 's excellent videos on magit, and I can’t seem to figure out the answer to this particular usecase.

Further digging leads me to this bug that’s been resolved since 2013, so that’s not particularly useful… ediff-merge during rebase shows empty buffers · Issue #916 · magit/magit · GitHub

But it does seem somewhat related; the changes I want to rebase are (obviously) unmerged.