Fade to white not working well

Hi.

When I add pictures to illustrate my video, I like them to appear with a flash of light, and use the "fade to white" transition effect. It usually works fine. But now, I'm having an issue with it.

Usually, the first frame (of the picture I've added to the video) is perfectly white. Now, I can perfectly see the video clip that's underneath, although... it's underneath. At best, covered with a white veil (I'm still talking about the first frame). And there's no fade to white per se, it fades to... opacity 50% at best.

Is this a known issue ? I'd be very grateful if someone could... show me the light.