SQLite GROUP BY

I am using DB Browser for SQLite to extract some interesting data from my database but I encountered one big problem with GROUP BY statement. Even the most basic SELECT I can imagine is not working properly.

(Filename nvarchar(2147483647)) SELECT FileName FROM TableName WHERE FileName LIKE '%Nieminen%' GROUP BY FileName gives 5 rows even though I know that there are 9 distinct FileNames containing the phrase '%Nieminen%' (I've browsed it).

Can it be possible that GROUP BY in sqlite compares only N (e.g. 10) initial characters? From my observation it might be true...

Any clues?