“what is a Unicode U+FEFF character? The client isn’t generating this themselves and it isn’t visible on the file itself. Do you [have] any guesses on how this is being generated?”

Aha!
https://www.freecodecamp.org/news/a-quick-tale-about-feff-the-invisible-character-cd25cd4630e7/

U+FEFF is a “Zero Width No-Break Space” but it’s also a byte order mark (for UTF-16?) if at the beginning of the file.