I imported a PNG image with a transparent background on top of a video. On the preview panel, everything is showing correctly. I can see my image AND the video underneath.
However, when rendering with default settings and with H.264 High Profile, my whole render is white. (I tried several times with different videos and different images but the same issue occurs)
I installed the snap version and I have the same issue unfortunately.
I found a solution tho. I exported the video without the transparent image then created a new project with the rendered video and image. It worked.
Hint: To make black border overlay, just use a black color clip with at Mask: Simple Shape filter on the top track. You can animate it with keyframe to make it apear over time