Well, I didn't want to downgrade emacs for a simple formatting problem, but I suppose that's my option as I wait for a better answer.
I didn't find a repo for 20.04 with old emacs 25, so I built it from source. Here's what I did, in case you or I have to do it again:
First, remove apt-installed emacs:
sudo apt-get remove --purge emacs emacs-gtk
Download http://mirrors.ocf.berkeley.edu/gnu/emacs/emacs-25.2.tar.gz - personally I untarred it in
/opt/ which may require sudo or chown
./configure complained about missing support for gif, tiff, and xpm -- don't care, build without.
./configure --with-xpm=no --with-gif=no --with-tiff=no
Finally, symlink this to
/usr/bin/emacs so it's available system-wide:
sudo ln -s /opt/emacs-25.2/src/emacs /usr/bin/emacs
And voila, the formatting works again, highlighting single-or-double quoted strings the same: