I got a new "Wacon One" tablet two weeks ago - my PC is a recent Fedora version (fedora 31), and as expected, it worked out of the box in testing and for many apps.
But MyPaint was the first GTK+3 app I tried with it, and it does not "see" the tablet, the Edit->Edit Preferences->Devices tab keeps showing only the Core pointer, with no pressure input available.
It turns out Fedora sets the GDK_CORE_DEVICE_EVENTS, which creates a single 'core input device' for GTK3, rendering the tablet "invisible" to applications.
The solution to use the tablet is simply unset the variable before calling any GTK3 application (or, at least, it worked with MyPaint) (unset GDK_CORE_DEVICE_EVENTS)
The variable is set in the /etc/xdg/plasma-workspace/env/gtk3_scrolling.sh file.
The reference for the fix is this github comment:
https://github.com/mypaint/mypaint/issues/388#issuecomment-317124496 - I am reposting the information here to improve find-ability.