A bug first seen on myltiple versions of MyPaint, but also seen on the GTK3 demo
https://github.com/mypaint/mypaint/wiki/Debugging-tablet-issues-on-Windows#testing-with-gtk3-demo

It only works correctly with pen on multiple monitors if win 10 is set for the SAME SCALING on both monitors (have not tested it with more than two)

It gets the pen position wrong (using the wrong scaling I assume). Mouse or finger positioning works correct.
I'm on a 14" 2560x1400 (almost QHD) touch and pen enabled laptop [Lenovo X1 Yoga, type 20FQ] set for 125% 
with an external secondary non touch 40" 3840x2160 (QFHD/UHD) display [Philips bdm4065uc] set for 100%.
If both are set to the same e.g. 100% or 125% pen positioning works just fine

The pen driver is by Lenovo, version "1.82.04.01 built by: WinDDk", and is called ApsHM64.sys

The scaling is done from the lower left of the screen.
It does not matter which display I set for primary/secondary
if the scaling factor is the same on both displays everything is OK
If the scaling factor on my pen display is LARGER than on the secondary, then the actually point addressed is further away from the lower left by the RATIO of the two scaling factors, (both X and Y).
If the scaling factor on my pen display is SMALLER, then X only(!!!!) gets closer to the left edge, I can't figure out by what factor, but dependant on both scaling factors, but not JUST the ratio... (Y is correct though!)

All works perfectly correct with both mouse and finger touch. And other pen programs works just fine.

Note in both, the pen-marker it self when close to or touching the display is correct. It is the cross-hair (and pressure marker) that is placed wrong.

I noted something REALLY strange, that might help debugging:
If I change the scaling to the same on both monitors, and test that everything works, and then change the scale on one, and hover the pen over the test-windows WITHOUT touching the display, the cross-hair is positioned correctly under the pen(!) until I touch the display, then it jumps.
So the engine CAN do it correct, and does so until first pen-touch!!

So have a look on whatever happens at touch, that might mess up things....
Both pen-touch, finger-touch and mouse-click trigger the error

It seems that the mingw ONLY displays "gdk_input_other_event:" AFTER the first touch, but updates the cross-hair correctly BEFORE that.
And the event displays physical (not logical) pixel coordinates within the test-window. e.g.:

   WINTAB motion: 3154.97,1304.04
   gdk_input_other_event: window=00000000006e14fc +2521+1316

so it seems that it scales things that should not be scaled. (or things are scaled more than once)
But note that the behaviour is different in X and Y when the pen display got the smallest scaling factor, here Y is correct and X is wrong.