It is your custom format, and not FileBot default behaviour, that is responsible for modifying the original title value:
Code: Select all
"VIII".lowerTrail() // result: Viii

Best to use
{kodi} or
{plex} as explained by kim. The latter is default. The former might be more suitable for you if you prefer SxE patterns.

If you choose to use custom formats, then it's best to use the GUI for rapid prototyping, as you would immediately see what a format expression does step by step as you write it, and issues like this would have been immediately obvious.
