as always I'm not sure whether this is a bug or a user error.
I got this background line that is masked so it doesn't intersect with the foreground object ('Reveal all' on the group, then 'Subtract' on the mask layer).
As you can see it works as it should in the viewport...

... in the preview render (CTRL-R) ...

... but not in the final, exported render (neither image sequence nor MP4):

An idea why this is?
Thanks,
Marc