Editing a filename in Finder will convert it to NFD
Even if the filename looks the same, it may be invisibly converted to a different sequence of bytes.
I noticed this while working on a file called Besźel.jpg.
The file had been written as Bes\xc5\xbael.jpg, but editing the filename in Finder would convert it to NFD.
>>> import unicodedata
>>> unicodedata.normalize('NFC', 'Besźel').encode('utf8')
b'Bes\xc5\xbael'
>>> unicodedata.normalize('NFD', 'Besźel').encode('utf8')
b'Besz\xcc\x81el'
Indeed, even interacting with the filename caused this conversion – I pressed enter to make the filename selectable, pressed ⌘+C to copy everything but the file extension, then pressed enter to finish. This triggered it! I was confused because the filename looked the same – I hadn’t made any visible changes, but the sequence of bytes had changed.
Once you know this is a thing, the fact that Finder converts to NFD is easy to find online – but it took me a minute or so to work out what was happening with this file.
I removed the special character from the filename rather than try to get it working; exactitude wasn’t important for my particular problem.