Please note when the Class field in the Main Line level is populated, the Item Line level will be populated with the Class value from the Main Line.
You will see the auto-population after saving the record and not upon creation.
If the Item record has a Class value in the Item record, this will take precedence over the Class populated on the Main Line level. Meaning, the Item Line Level Class field will be populated with the Class value of the Item record instead of the Class in the Main Line (if the values are different).
However. if the Vendor Bill is created from a Purchase Order, then it would just inherit what's on the purchase order. If the Class field is not on the Purchase Order, then the Bill wouldn't have Class field populated as well.