SAP_ALL is authority, but authority does not control which fields are available - this is done by customizing of field selection, which is for most people quite difficult to understand, as there is a field selection for industry, for material type reference, by transaction and by plant and they interact.
In general you cannot expect to maintain a field with a BAPI if you can't maintain it online in the normal maintenance transactions.
If that field is not included in a field selection group then you wont see it on a screen and can't maintain it.
If the field is assigned to a status E then you can only update that field if you defined to update the purchasing view in your BAPI, even it is visible somewhere else in your material master.