Yes, filenames with accents should work just fine.
Yes, it looks like a bug.
Check Media doesn’t report 1 < 2.mp3 as missing or unused but mpv is looking for 1 < 2.mp3 and there’s no sound.
Check Media reports a & b.mp3 as unused and a & b.mp3 as missing but mpv is looking for a & b.mp3 and plays it just fine.
Strangely, while adding the audio file via drag-and-drop, a & b.mp3 was correctly HTML escaped and saved as a & b.mp3 in the card, but it didn’t happen to 1 < 2.mp3 and it was saved as it is.