Hi Marscaleb,
Thank you for reaching Microsoft Community and apologies for the inconvenience this has caused you. We appreciate the time and effort spent in troubleshooting the issue prior to posting and in providing the video.
I noticed on the video that the device was connected to power, does the issue also occurs when running on battery?
Also, how about on other applications, do you also experience this issue when drawing diagonal lines?
I'm not sure if this was tried already since this was not mentioned on the previous post also but if not, kindly try this below:
Open Device Manager
Expand Human Interface devices
Right-click on Intel Precise Touch
Select Uninstall device
Note: If prompted with a checkbox stating "Delete the driver software for this device", leave it uncheck then hit Uninstall.
See if Surface Touch Pen Processor is listed as well and do the same step
Do the same step also for the Intel(R) Iris(R) Plus Graphics under Display adapters
Restart the device after and see if issue persists
Let me know how it goes.
Kind regards,
Marrion