This is because the center point of your models do not expect a foreground/background boundary. By putting a hole at the center, the boundaries seek to stabiliz

This is because the center point of your models do not expect a foreground/background boundary. By putting a hole at the center, the boundaries seek to stabilize the system by bubbling outward. The best way to put it is that there is no stable state where a doughnut hole exists at the center. An interference pattern ripples outward and destroys the system.