I don't think it's because the header forum is hidden. There are no separate permissions for "seeing" the header. The header forum is hidden on most boards and on this site it is working fine for guests. It's possible that the auto-template edit isn't working all the time. Do you have any template edits that modify the threadslist table? It's also possible the header is not being associated with that forum correctly.
However, I still find it very strange that you are able to see the Header as an admin. Let me know once you have updated to the new version. Forums headers have more stable connections in VaultWiki 3.