I checked out the slugify function in django-autoslug - same thing happens.
I know just enough about Unicode to know that lower-case i with dot is a different character than lower-case i without dot, and that the two are not the same.
From the slugify function docs, it does say that any characters that don’t map to ascii characters are removed. This appears to be the case here - there is no ascii mapping for the lower-case i without dot, and so it’s dropped.
I don’t believe there is a valid work-around for this without doing your own character substitution on the string before using slugify.