Hi,
The material master field selection works based on the maintenance status for the field (OMSR). This will make the field mandatory when you open any view maintained in the maintenance status for the field.
If you want the field to be mandatory only in costing view, the easiest option would be to make the field as optional in field selection and create a transaction variant in SHD0 for MM01 and MM02 and make the field as mandatory in costing view only.
Regards,
AKPT