So I was having the same issue. In the screenshot, You see the end of Vas Lab scene and the beginning of the PSB Reveal clip.
I was attempting to add the dip to white to the end of the Vas Lab clip then butting up against the PSB Reveal clip which has a white background. What resulted was a dip to white, then to black which was bad transition (white to black to white). What I did to fix this was butt the scene and the clip together, then add the Dip to White transition to the beginning of the PSB reveal clip, then set the Alignment of the transition to End at Cut. This then allowed me to adjust the Duration back from the cut which dipped the Vas Lab scene to white right into the PSB Reveal clip! See below: