I'm using Kdenlive 19.12.3 which came with my Kubuntu 20.4.

When I create my own videos (with default project settings), I have not had any problems rendering.

When I import another video for adding text subtitles to it, sometimes it works OK, but other times it replaces portions of the video with green #004d00 where the text clip shows huge with horizontal stripes.

Is this a known fault? Can anything be done to work around it?

If I upgrade to a newer version - 21.08.3 seems to be available in the software centre - is there a way of keeping the old version so I can choose which one to use?

you can use any OLD version using APPIMAGES. Kdenlive devs has fixed a lot of bugs on latest 2 years and use an old version of Kdenlive is just a way to have random issues... if you are worried about OLD projects, just do not overwrite them using a newest version of Kdenlive.