Lead becomes disconnected from horses after period of time

My sister has horses on lead tied to a fence post but whenever Me or my Sister go far away from her horses the lead is either gone or is on the ground and causes the horses to become lose.

The first two images show before the lead is gone and it is only visible on the fence post from one angle, the lead is on the horses but is invisible.

The Third image shows that the lead is visible for now.

The final image shows the lead on the ground and not on the horses