Yes the users have permissions to view the docs but I don't think that it would affect this as this as I'm not using the document library and this is just an internal variable in the workflow. What's happening is the Attachment is an input to a linked model.
When entering the model for the first time the Attachment is displayed correctly on a Download List control but when it comes out it displays blank but with the correct count. I checked the mapping and even replaced the End component and remapped but it's still happening. It's like the mapping is corrupt when going back to the parent model and it's something I've never seen before.