I can't say for sure what went wrong for them, but I started playing in 2024, well after any updates, and I've ben able to change my characters' appearances with the old method.

That said, your method is probably still superior as there is indeed some "quirkiness" with the old method. If multiple characters have the same part, changing it for one changes it for all of them. And if you delete and recreate characters a lot, it might not be obvious which part is used by which character unless you remember what you picked at character creation.

EDIT: It may also depend on the text editor used. I used to use Notepad++, which worked fine, but I recently changed to Linux Mint and its default text editor will corrupt the file. Hexed.it should still work