Hey Michael, thanks !
If I do like you say, the mask does not follow as you can see (3rd screen, at the bottom).
I’ve been told I should Render in Place the clip with the mask before moving it.
But now the cut out mask appear all black if I don’t have a clip under it. Screen “Capture d’écran”.
When there is a clip under it, the cut out mask appear, but the clip underneath it doesn’t appear in the black left by the cut out, as it’s supposed to. Screen “2”.