Another day, another "Huh, why does it look like <unexpected situation> happened, that can't be right. What's wrong with my code that makes it look like that?" answered with "Your code is working exactly as designed and <unexpected situation> did, in fact, happen"
details on this <unexpected situation>, arcane git nonsense, jargon
As CW'd, today's situation is particularly arcane git nonsense, so you have my sympathy regarding any part of this that makes sense to you. But for the record:
I'm building a monorepo out of git subtrees, and my rewritten history showed no changes after squash-merging in a branch from a certain repo that itself I have massively rewritten history in. It was almost as if the branch was even with development. I was doing a lot of rewriting the squashed message to clean up a bunch of noise, so I figured I'd just broken something in that cleanup and cleaned up more than I should.
But no, turns out I'd just screwed up in creating the alternate-history version of this one branch, and it was indeed just...even with development, and thus had no new changes, so when my code dutifully removed all the redundant change messages, I correctly ended up with an empty commit message 
