I've just change my Premiere from Mac (CC 2015) to PC (CC 2018). in a new Project, I've import on track 2 (video on tack 1) a png image 3000X3000 (1,0 pixel ratio) in a DVCPro Sequence (1,5 pixel ratio). When i'm scaling down the image, the video disapear and i can see the background. It does'nt seam to handle my images in 1,0 ratio. I've the change the image in photoshop for 1,5 ratio and it's working.
But I need a more efficient solution, cuz I cannont change all my image.