After a bit of tinkering, my device appeared also as a pointer as “Network Tablet Pen (0)” . Using it
xinput map-to-output “$( xinput list --id-only “Network Tablet Pen (0)” )” HDMI-0
worked like a charm
I had the same symptoms. Try the changes to the code I made in pull request 95. It uses a newer Linux API to register the device, and libinput seems happy with it now, so it works in gimp and inkscape.