Latest Kdenlive on Manjaro stable (21.04.0) seems to have some problem with rendering the transitions.
All seems ok in the preview/editor window but when creating the final rendering, the transitions are not applied and on scene skip to another in a harsh way.
Tested on Xfce, using a mp4 (x264) rendering with different parameters.
Same project rendered on Flatpak Kdenlive 21.04.1 does not exhibit this bug.
The bug could also resides in MLT rather than Kdenlive itself.
Previous Manjaro Kdenlive release did not have this bug.