The chest is a really common area for cockatoos to over-preen. Under the wings can also be a problem area.
I know yours is a tiel, but that doesn't look like molting to me (although I am not sure it is plucking).
In a molt, you shouldn't see bald patches of skin really. Disease, allergies etc can also cause this.
Have you had a blood panel to check kidney/liver function? There are also fungal and bacterial infections of the skin that can cause similar conditions.