On any page (e.g. one called Source Note) create the following text [[Target Note]] where Target Note does not already exist. Do not click on the link to create the page. Then create the page manually in the repository.
Open Target Note and look at the backlinks. No backlinks are shown either linked or unlinked. Open Source Note and click on the link. This correctly takes you to Target Note but still no backlinks are shown for Target Note.
If you then create a new page and link to Target Note again this will create a single backlink to Target note for the new page but still won’t register the backlink to the original Source Note.
Finally if you reopen the original Source Note and make any edit it will finally create the second backlink for Target Note.
