Because "commit" pretends as if it took the exact commit object name to be fixed up (after all, it accepts "yyyyyy" that is a name of the commit object), it would be nice if the fixup is applied to that exact commit, even if you had many commits that share exactly the same title (i.e. not just shared prefix).
Unfortunately, "rebase -i --autosquash" reorders the entries by identifying the commit by its title, and it goes with prefix match so that fix-up commits created without using --fixup option but manually records the title's prefix substring can also work.
We could argue that the logic should notice that there is one exact match and another non-exact prefix match and favor the former, and certainly such a change would make your made-up example (you didn't actually have a commit whose title is literally "This is prefix") above work better.
But I am not sure if adding such heuristics would really help in general. It would not help those whose commits are mostly titled ultra-vaguely, like "fix", "bugfix", "docfix", etc.
Another possibility is to teach "commit --fixup" to cast exact commit object name in stone. That certainly would solve your immediate problem, but it has a grave negative impact when the user rebases the same topic many times (which happens often).
For example, I may have a series of commits A and B, notice that A could be done a bit better and have "fixup A" on top, build a new commit C on it, and then realize that the next step (i.e. D) would need support from a newer codebase than where I started (i.e. A^).
At that point, I would have a 4-commit series (A, B, "fixup A", and C), and I would rebase them on top of something newer. I am undecided if that "fixup A" is really an improvement or unnecessary, when I do this, but I do know that I want to build the series on top of a different commit. So I do rebase without --autosquash (I would probably rebase without --interactive for this one).
Then I keep working and add a new commit D on top. After all that, I would have a more-or-less completed series and would be ready to re-assess the whole series. I perhaps decide that "fixup A" is really an improvement. And then I would "rebase -i" to squash the fix-up into A.
But notice that at this point, what we are calling A has different object name than the original A the fixup was written for, because we rebased once on top of a newer codebase. That commit can still be identified by its title, but not with its original commit object name.
I think that is why "commit --fixup <commit>" turns the commit object name (your "yyyyyy") into a string (your "This is prefix") and that is a reasonable design decision [*1*].
So from that point of view, if we were to address your issue, it should happen in "rebase -i --autosquash" side, not "commit --fixup" side.
Let's hear from some of those (Cc'ed) who were involved in an earlier --autosquash thread.
https://public-inbox.org/git/cover.1259934977.git.mhagger@alum.mit.edu/
[Footnote]
*1* "rebase -i --autosquash" does understand "fixup! yyyyyy", so if
you are willing to accept the consequence of not being able to
rebase twice, you could instead do $ git commit -m "fixup! yyyyyy"
I would think.
