My cockatiel has a tiny little bald spot on her chest, it’s hard to see at times but it is there. Right now, she preens

Are you able to send a photo showing where on her chest you see this? You can attach photos to this conversation.

Birds grow feathers in tracts and there are unfeathered tracts of skin called apterae, also. Sometimes when a bird has bathed or is preening or moving in certain ways the apterae can be seen. There's an apterae down the center of the breast. If the bald patch becomes visible all the time it would mean she has lost some feathers to expose the skin. That can mean she has an infection making her feel itchy. It can be a Candida yeast overgrowth,  giardia, hexamita, or avian gastric yeast  which are common in young cockatiels.  It can also be things like allergies or hypersensitivity to something she's exposed to. So if the bald patch becomes visible all the time you should get her examined by a veterinarian familiar with birds.