Custom Function not getting package details when triggered from Workflow Rules.

Custom Function not getting package details when triggered from Workflow Rules.

I have a custom function for Packages that submits a form in our Creator app that we use to generate custom shipping labels (internal staff complete deliveries so we cannot generate shipping labels straight from Inventory). When the function is executed during testing directly from Workflow Actions > Custom Functions all of the package data is obtained.
When the function is triggered as part of a Workflow rule in packages, the shipping address information is missing. 

Package details returned:
customerName = package.get("customer_name");
customerEmail = package.get("email");
salesOrderId = package.get("salesorder_id");
salesOrderNumber = package.get("salesorder_number");
orderDate = package.get("created_time");

Package details missing:
customerPhone = package.get("shipping_address").get("phone");
shipAddress1 = package.get("shipping_address").get("address");
shipCity = package.get("shipping_address").get("city");
shipState = package.get("shipping_address").get("state");
shipPostCode = package.get("shipping_address").get("zip");

I'm guessing it's got something to do with the fact that it's all related to the shipping_address information. Has anyone come across this before?