Kdenlive currently creates all transitions as automatic transitions. Automatic transitions resize with the clip they're attached to, and they die with their clips when the clips are removed from the timeline.

Please add a new toggle button to Kdenlive's bottom status bar, next to the existing toggles for "split audio from video", et cetera. This new toggle should be labeled "automatic transitions". When set, all newly created transitions get their "automatic" property set. When unset, newly created transitions get created with their "automatic" property reset.