I've noted that this issue is rather inconsistent. While I do not experience it myself, my friend has found it very difficult to play because the window fails to reset his mouse's position to the center of the window (Edit for clarity: As with the original poster, the mouse will exit the bottom of the window when looking down). This issue is still evident for him on fullscreen mode, though I am unsure if the original poster of this issue is still experiencing it with fullscreen.
I've noted that this issue is rather inconsistent. While I do not experience it myself, my friend has found it very difficult to play because the window fails to reset his mouse's position to the center of the window (Edit for clarity: As with the original poster, the mouse will exit the bottom of the window when looking down). This issue is still evident for him on fullscreen mode, though I am unsure if the original poster of this issue is still experiencing it with fullscreen.