Take an existing project, render part of the timeline. After that's done, use the spacer tool to shuffle everything in the timeline to some later part. The rendered timeline doesn't get invalided. Reproducible: Always
Can confirm this still happens The work around is to put a something, like a title clip, and stretch it. Once it turns red, remove it
I see this too, with 18.04.3.
It would be nice for single-track spacer to invalidate preview regions and all-track spacer to shift preview regions to above a re-render of the preview. Confirmed in 18.12.1b.
It seems that this report is related to a very old unmaintained version. A lot changed since then and in my experience it is fixed. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)