Just an update, I did see the other thread that had the same issue (Graphical problem on ubuntu Hirsute with mono) but my issue never went away. I had done a few updates of Ubuntu since then, and updated MP and the problem remained. Until today: I changed my desktop to the Ubuntu Budgie 21.10 and that fixed the problem. Suggests to me the problem is something to do with the desktop environment and how it interfaces with MP. Anyhow, I’m just happy to have it all working again.