The issue is that the "release" function appears to be called before the "drag" function for the last coordinate. The fix is probably only lowering the pen when the mouse button is first clicked, as this is guaranteed to be called before "release". I ended up with something like the following (after removing the pen lower logic from the "release" function):

def down(a, b):
    print("down")
    p.goto(a, b)
    p.down()
bg.onclick(down)