Magmi Not Updating a specific System Attribute but Updating All Others

[Sparetoolparts]

// Provide a general summary of the issue in the Title above:

I am using Magmi for my Magento 2 website and have been updating my products successfully until now. I need to update an specific attribute which is var/char with a dropdown menu. It has been set as System Attribute by the developers of the plugin it belongs to. When I try to update this attribute it does nothing.

I've exported a sample product from the site and I note that this attribute "am_shipping_type" is appearing under a column called "additional_attributes" the attribute I need to update is showing like this in the column am_shipping_type=Small Parcel

Does this mean If I want to update this attribute in Magmi I need a column in my .csv called "additional_attributes" and I need to define the change like this am_shipping_type=Large Parcel

attached is my file I'm using for my update in Mamgi

Milwaukee_test.xlsx

[Henrik41]

Same issue here. After adding the additional_attributes column the attribute content is not updated.