I have a project with several effects on tracks, ("Transform", "Crop, Scale and Tilt") and "Position and zoom" on master. After today system upgrade Kdenlive started to use mlt 7, rather than 6 before. Since then "Position and zoom" effect is not rendered (despite it looks OK in Kdenlive preview window). When I changed the settings in Kdenlive to use mlt 6, the videos are rendered properly again.

Up to version 21.04.3 Kdenlive is using MLT 6. With version 21.08 Kdenlive will use MLT 7. We are still testing as this change needs some updates of the source code. So use MLT 6.