[Cin] Fade black flash

Hello, when I put a fade in the title video effect (here the white "Hello" text https://www.youtube.com/watch?v=dJLj0w6NUSo) you can see that letters appears as a black flash briefly in the first few frames, then goes back to transparent, then the fade starts normally with the white color. Does anyone know how to fix this? I have the same problem when using transparent PNG images with fade video transitions, first black, then normal transparency. Thank you very much, Bests, Camille.