Yes coring is a defect. Cited structure are common and usually removed during homogenizing heat treatment.

Actually it occurs due to non equilibrium cooling conditions.  Think as liquid solidifying into solid, now solid diffuse slower than liquid and in an alloy this process occurs for a range of temperature. If we don’t maintain sufficiently slower cooling rate then what happens.

When we cool and after the first solid of say of composition a forms. After further undercooling liquid solidifies further but due to slow diffusion of solid a it’s centers is still there and new solid b composition forms radially around it. In this ways when the solidification is completed the center of solid has region of high melting( remember this solid formed at higher temperature) and outer region has low melting solid. This type of structure is called Cored structure or coring.

Hope you got my point you can further refer to books like vijender singh or callister for further reading.