I have the same problem DX
This happens to me after the restart of my laptop.
Since it happened I have uninstalled every wacom driver and SAI 2, restarted my laptop, redownloaded the program and the drivers, reinstalled them. Multiple times.
At first, when I reinstalled the driver, everything worked on sai. But when I restart my laptop again, this thing straight line before the curve starts, happens again.
I have a Wacom Intuos CTH 680 and installed the compatible driver 6.3.29-6 (this is not the latest driver). I am running a windows 10 on my laptop, and I already checked that I'm not using windows ink and that the press and hold feature in the control panel (under Pen and Touch) is not enabled.
Also i tried stopping the wacom tablet service and restarting it from the services panel
Anyone has an idea of what is happening and how can i fix this? ;D;
Thank you so much ;;