It looks like an error. It could be a planchet error, such as an occluded gas bubble. If so, try pressing gently on it with the tip of a toothpick while observing it under a microscope. If it flexes, then that would indicate a gas bubble. It could also be an incipient lamination that hasn't split open. It could also be a die error, specifically a die dent. Hard to tell from here.
Error coin writer and researcher.