Update: this bug is tracked as an Ubuntu issue here: Bug #2061079 “GTK-ngl (new default backend) rendering issues wit...” : Bugs : gtk4 package : Ubuntu
It’s a problem with the 470 driver that causes GTK apps to fail to render text correctly. Newer versions of the driver do not exhibit the problem. If (like me), you are unable to use a newer version because you have a legacy GPU, then there is a workaround: add GSK_RENDERER=gl to /etc/environment and reboot.