I'd chalk this one up to the reviewers not being careful. I could perhaps understand a rejection without a clear description of what the problem was. Many editors are not as diligent as you have been in writing a description of the edit. But even that is not the case.
There is no problem with the edit being unclear in the "rendered output" view, while being clear in the markdown view. Nor is your edit "too minor" (though that was not the reason chosen anyway), so I'm at a loss at trying to come up with a reason to reject it. And I'm usually pretty good at that.
I would have most certainly accepted it and think it should have been accepted. Luckily the edit has now been made. Should you bump into a similar problem next time, then don't hesitate to make a similar edit.