"Guess case" capitalizes the Spanish possessive adjective "mi" if "Uppercase Roman numerals" is checked.

This is somewhat similar to MBS-12523, concerning "MIX". That one was easier to fix since "mix" usually only appears as extra title information.

Maybe it'd be reasonable to exclude "mi" from Roman numeral capitalization if sentence case is selected (or if it isn't already capitalized?). I suspect that the Spanish word is more common than the Roman numerals, although my Lucene skills aren't good enough to test that theory.