Super thank you

The 'high' file works perfectly. It says DXV3 and in an alpha surrounding.
The 'normal' file has the same weirdness. In the preview it has a black background & when I trigger it the background is transparant. When I preview whilst it's triggered/playing, the black background is gone.
Also, I don't know the footage very well, but the preview is a line instead of text, is that normal?
I was looking at the clip properties & now noticed that even on your clips the alpha type in the video properties for the clip is on premultiplied! Putting it to straight there does make a difference on your clips & on some of mine (not all of them....)
Hope these screenshots are clear.