This is because there is no such a letter as “LATIN SMALL LETTER I WITH DOT ABOVE” while İ (U+0130) is “LATIN CAPITAL LETTER I WITH DOT ABOVE”. So linguistic casing will convert this letter to i (U+0069) LATIN SMALL LETTER I, but converting this small letter i to uppercase won’t go back to İ (U+0130).
I will support linguistic casing in future versions probably as an option.
Reference:
http://archives.miloush.net/michkap/archive/2004/12/11/279942.html