The NFC/NFD normalization does sound plausible, but I can't for the life of me think of why I didn't see this until now.

But I did just discover the culprit. At least I'm next to positive. MacDrive. A Windows driver for reading Mac disks.

Anyway, from my Mac, I put a file named "Pulsação" onto a thumbdrive formatted HFS+ When I view that folder in Windows, sure enough, the cedilla is next to the c, not directly under it.

I can in fact create another folder in Windows with the correct filename and it will happily sit along-side the original one. Because as far as Windows is concerned, they're different.

Move the thumbdrive back to my Mac and the folder names clash and one is now appended with ~1.

I did some copying/moving of folders to and from my NTFS drive under Windows and didn't run into any problems on the Mac however. So while I seem to have discovered that MacDrive is showing the folder names incorrectly when a cedilla is present, it doesn't cause an issue when using the HFS+ format on the Mac, regardless of where the file is created/named. I suspect the issue is only when viewing that file on an NTFS volume on the Mac - which I'll try in a moment by bringing the other drive over again.

Ideally I could just use HFS+ for my large media disk in Windows, but MacDrive has proven to me time and time again that it's unreliable by causing simple folder browsing to hang Explorer on my music drive.

So it seems like Songbird isn't responsible for this issue. It was a coincidence that I first noticed it on songs which it did mess up tags for (numerical genres, adding ID3v1 and forcing ID3v2.4 instead of preserving the previous 2.3)