Take Away: When you are working with Promoted Properties that may or may not be included inside your message make sure you set the Property Schema Base property to MessageContextPropertyBase.
The two values for this property are:
MessageDataPropertyBase (Default) – This means the data used to populate this field will come from a message
MessageContextPropertyBase – This means the data use to populate this field may or may not exist in a message. i.e. it could be prompted inside a custom pipeline and set to values not inside the message.