Ah that explains a lot, I was wondering why I kept seeing 2 Xs where there was obviously only a single beat. I still don't get how osu! fails to do them though -.-
Edit: I've tried and succeeded in creating one of those strange sliders. Simply go on the editor, put the Beat Snap Divisor on 1/8 and create the smallest slider possible. Then go on the Select tool and click on the first, original little white square of the slider (where it starts) and drag it towards the end of the slider (the opposite will not work) and there you go. I don't quite understand how to tap those sliders, I can only get 100s by tapping way too early, if I try to tap when I should I only get an X, I guess that's why osu! fails at those...