The game is not running in fullscreen. It is borderless windowed mode, the cursor will not stay in the screen - it jumps to second monitor.
Also causes some issues with performance as not true fullscreen.
Try to go to \Steam\steamapps\common\TreeOfSavior\release
find user.xml
FInd the line with Resolution mode
Change the value to 0.
If i remember it right, 0 = true full screen, 1 = windowed, 2 = borderless windowed.