I created a custom filter which basically does the same as what happens if "canFilterUnmatchedItems" is true.
In case of "canFilterUnmatchedItems == false" my custom filters works as expected when I type the first character but is not updated when I continue typing.
Having had a look into code I've seen that none of the custom filters are called again on TextChange. Only call occurs immediately after creating a new session.
If I set the property to true again, everything's as I expect. My custom filter is called each time I type another character.