That was the problem — installing from the Snap store. After I upgraded to Ubuntu 18.04 the Electrum client I already had installed would no longer come up, so I figured oh well I’ll just install it from Snap. I re-entered my seed phrase and all that.
Turns out my original ~/.electrum directory is still fine, and all I had to do to clean up everything was to go through the normal install. I first uninstalled the Snap version just in case, then:
sudo apt-get install python3-setuptools python3-pyqt5 python3-pip
# (nothing to do there, but still)
sudo pip3 install https://download.electrum.org/3.2.2/Electrum-3.2.2.tar.gz
Now I have 3.2.2 and Preferences works fine. Thanks, it’s beautiful.