Bug? Git won't apply a split hunk that went through a text editor

Hello, git enthusiasts,

I’d like to report what I suspect is a bug in Git, tested in 2.18 and 2.14. (I’d be delighted to be corrected if it is my own misunderstanding.) I’m reporting it here based on guidance from https://git-scm.com/community.

I created a minimal testcase with a detailed README here: https://github.com/philipmw/git-bugreport-2018-hunk-fail

Overview of the bug:

When interactively selecting hunks to apply, using `git checkout -p <tree> <file>`, git will correctly apply an unmodified hunk, but will refuse to apply a hunk that went through a text editor ("e" command), even when I made no changes in the text editor.

Thanks for any advice or attention you can give to this matter.

— Philip