@gez-mycroft Thank you for the reply! 
Originally, with Ubuntu 18.04, I installed using git. This installation carried over into Ubuntu 20.04. I first tried solving the issue with downgrading python from 3.8 to 3.7.5.
I also tried uninstalling and reinstalling using snap. That presented the same issue. I uninstalled again and reinstalled using git, which is the current installation. (I’m trying my best to maintain a single variable per trial, other than maintaining python 3.7.5 throughout.)
I checked the virtual environment python version, and it is 3.7.5. (That never crossed my mind to check, which seems so obvious now!)
I thought about wiping everything with a fresh Ubuntu 20.04 installation. But I don’t like not knowing what’s causing this problem. Plus, I suppose, there’s no guarantee that a fresh install will actually fix it.
Dan.