Seems like all of our problems are due to a problem with an earlier build of 20.2 Nibia. The simplest, most straightforward solution is to download the latest build image and reinstall (that’s what I’m going to do). See the PSA: KDE edition came partly with git-packages section of this post:
There is a script there which can remove the git packages and replace them with stable repository versions. This is a very unlucky thing to have happen right when I’m trying Manjaro for the first time.