I have noticed that when I put two “Transform” effects on top of each other, the result is that the element is displaced.

It is the same kind of displacement when using the “Preview” instead of “High Quality” compositing mode.

I also noticed that. And sometimes clips simply isn't rendered if there are overlapping transforms.

i opened an issue about this some time ago: https://invent.kde.org/kde/kdenlive/issues/364