SUMMARY
(I am posting this bug report under advisement from Jason Gerecke on linuxwacom-discuss).

A friend of mine has recently switched to Arch Linux (with KDE Plasma as his desktop) and is using a Cintiq 13HD for drawing with Krita. He has found that after calibrating his tablet, the offset between the cursor and the stylus increases the further away from the center of the drawing area he moves the stylus (at it's worst on the very edge).

He currently runs a dual monitor setup on his Nvidia GTX 1050 Ti, however he has also tried recalibrating his Cintiq without the second display connected. Without the second display connected, the offset is worse (he performs his calibrations through the Wacom Tablet KCM in Plasma Settings). This issue persists both inside and outside of Krita (other drawing programs, empty desktop etc).

After discussion with Jason Gerecke on linuxwacom-discuss, Jason has advised that the error is not in the xf86-input-wacom driver but in the wacomtablet KCM module. He has written a preliminary patch (which I will post here once I have obtained his permission) which does appear to partially correct the issue from our testing. The patch changes the calculation of the tablet area to take into account the tablet sensor sizes being slightly larger than the Cintiq display itself.

STEPS TO REPRODUCE
1. Configure a Cintiq 13HD or equivalent tablet display to the below settings using the wacom KCM module.
2. Run Krita, GIMP or a program that recognises stylus input.
3. Draw from the center outward, taking note of the increased offset as the stylus approaches the edge.

OBSERVED RESULT
The offset between the cursor and the stylus position increases the further away from the center of the drawing area you become.

EXPECTED RESULT
The offset would remain consistent regardless of the stylus location in the drawing area.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.58.0
Qt Version: 5.12.3
Kernel Version: 5.0.13
Xorg Version: 1.20.4
Wacom Xorg Driver Version: 0.36.1
Nvidia Driver Version: 418.74
Wacom Settings KCM Version: 1:3.1.1

ADDITIONAL INFORMATION
Cintiq Model: Cintiq 13HD
- Hardware IDs: 056a:0304
- Connection Type: HDMI
- Resolution: 1920x1080
Second Display Resolution: 2560x1080

Cintiq Calibration (dual display, default):
- Map Screen To: Tablet Area
- Tuning Type: Fine
- Horizontal: 373, 58730
- Vertical: 336, 33150