manageItem API doesn't remove the item additional attribute values that are not passed in input
If some item attributes are modified and are passed to manageItem API, it only overrides the fields that are passed. But keeps the other fields intact without
Values of the additional item attributes are not deleted since the delete operation was not passed as part of the Additional Attribute tag in the input xml to manageItem API.
Resolving the problem
To reset the item additional attributes pass Reset= "Y" in the AdditionalAttributeList tag in the manageItem input API.
This will override the attribute values passed in the API input and delete the attributes that are not passed.