19 and 18 are out of order, but computers don’t sort Roman numerals numerically unless you make them. They’re sorted correctly alphabetically.

Would you mind sharing the revelation? I’m not sure yet what’s infuriating. The files are sorted by year primarily; the only tie is 1950, and “I” (in “XIX”) comes before “V” (in “XVIII”) alphabetically, so they’re in order as well.