Final Cut Pro - export with transparent background transition on the end

Hi Guys! I have issues with transparent background exports. I know how to export a transparent background but what I hardly never can manage is to use a transition effect at the end of the footage and export it with a transparent end. Sometimes, I don't know how, but I can export with a simple dissolve effect at the end, and it dissolves to transparent which I really want. But if I use a wipe, or something mose complicated effect there are some black frames, and after the transparent. Help me please how to manage it? Or is that a bug?