The only time FFF sets title_sort instead of just title is when you have 'Force Title into Title Sort?' checked. (Or "New Only" checked for Title which preserves existing title_sort by setting it to the 'old' value.)
However, it looks like Calibre only updates title_sort if title is actually
changed. So if you
already have a title_sort value that doesn't match Calibre's algorithm applied to title, you won't get it from an FFF update unless title has changed.
I don't see anything wrong with FFF here.