This almost always means the file is saved in an older Windows encoding (commonly Windows-1252 or Latin-1) rather than UTF-8 — Excel opens both without complaint, which is why it looks fine there. To fix it:
- Open the CSV in a plain text editor (Notepad++, VS Code, or even TextEdit on Mac).
- Use “Save As” / “Reopen with encoding” and explicitly choose UTF-8 (not UTF-8 with BOM, if your editor distinguishes).
- Re-upload the re-saved file.
This is especially common with lists that have accented names or non-English addresses, since those are the characters that get mangled by the encoding mismatch.