How do I delete undefined pages created as a byproduct of moving pages from one space to another?

[Deleted user]

I moved pages from a first space to a second space.  Afterwards, I noticed that there are undefined pages in the first space that are linked to from the pages in the second space.  I am unable to delete the undefined pages.  When I inspect the pages in the second space, I cannot find any links to the undefined pages in the first space.  When I edit the pages in the second space (no matter how small or inconsequential the edit), the corresponding undefined pages in the first space disappear.  I am using Confluence server 6.15.2.

First, why do undefined pages appear after I have moved a page from a first space to a second space?

Second, why am I unable to delete the undefined pages directly?

Third, is there a better way to get rid of the undefined pages than by manually editing, one at a time, every page in the second space from which an undefined page is allegedly linked?