Still just a guess.
From your screenshot that is such a large offset that I suspect Gimp is still working in landscape mode. Something like this:
If you feel up to experimenting you can rename (as a backup) the devicerc file in your Gimp profile.
Stay in Portrait screen orientation, start Gimp and Gimp will make a new default devicerc file. Your old settings are no longer there, go in to Edit -> Devices and put the tablet pen into screen mode. Maybe Gimp will adjust to the new orientation. Might/might-not prove something and of course might be something completely different.
From your screenshot that is such a large offset that I suspect Gimp is still working in landscape mode. Something like this:
If you feel up to experimenting you can rename (as a backup) the devicerc file in your Gimp profile.
Stay in Portrait screen orientation, start Gimp and Gimp will make a new default devicerc file. Your old settings are no longer there, go in to Edit -> Devices and put the tablet pen into screen mode. Maybe Gimp will adjust to the new orientation. Might/might-not prove something and of course might be something completely different.