I have some .mov files that already have a transparent background. They worked fine in Kdenlive version 20.04.1. The transparent background automatically worked.

I upgraded to version 20.12.3 and now those files show a black background. Why? How can I correct this?

Tried right now.. with a MOV with alpha channel.. and everything is ok, the alpha channel works perfectly fine.

The background is displayed in black in the preview, but if I add another videotrack under the one containing the mov file with alpha I can see it's transparent where it should be and I can see the clip below, the alpha channel is perfecly taken into account