Unable to export Signature Export custom field using Document Generator

Description

When we try to export the Signature Export custom field from the eSign Electronic Signatures app, this comes out blank. This field was previously being exported. 

Pre-conditions
Have eSign Electronic Signatures app installed and have a signature in a issue (like a test)

How to reproduce

1. Create a template to export this specific field - ${Signature Export}
2. Add it to the Doc gen templates
3. Export the issue with the signature

Actual result/Attachments

The field comes blank

Expected result

The field should come with all the signature information (see output file)

Workaround
Use the Xporter app