FAQ
Q.
Lines have been observed on the surface of some MLZ2012-type products. Is this a problem?
  • Inductors (Coils)
  • >
  • SMD / SMT Inductors (Coils)

The line in question is a deliberate feature of the product's design. 
In order to realize the required magnetic properties, different magnetic materials are layered together. 
Consequently, this interface is visible as a line in the final product.