SharePoint Document Library Lookup Column is not Appearing in Document Properties

I have a document library with a lookup column.  In this library I have a document that I am setting up to be a template (its just .docx right now) and I want to include the lookup column as a field in the document using the "Insert>Quick Parts>Document Property", but when I do that the lookup column is not listed as one of the fields.  This is my problem.

I know this is possible because the lookup field was showing up as a document property at one point, but then, as I further tweaked the library and document, the field has disappeared from the list of properties.

Any ideas on why this disappeared and how to get it back?

Thanks in advance!