hello.
I have a project template to edit all the news we use in out local newscast, so i had all the titles, all the effects, the exporting formats and paths preconfigured.

Now with latest appimage (21.04.3) on Ubuntu Linux 20.04, i can edit everything fine for the first clip, but after i exported the clip, and after i remove all the footage form the main bin, loading new footage and etiting it, results that i cannot playback main timeline, but ONLY the clip monitor (spacebar).

If i press the "play" button under the project monitor, i get only less then a second of playback for every button press.

So i had to close the project and reload it back.

Hope it could be usefull to track the bug

You could save your assets instead. Titles and effects as well as effect stacks can be saved, and you can save sequences to library, and then expand them and edit each item to your liking.
Another take could be not saving your project or saving it under a different name, and then reopen the project or even kdenlive for each new story.