List of steps to reproduce (step by step, including full links if applicable):

• See :vi:w:Special:PrefixIndex/WP and :vi:w:Special:PrefixIndex/Talk:WP
• Try accessing one WP: or Thảo luận:WP: pages (ones with colon)

What happens?:

• WP: ones: You arrived at Wikipedia:Foo instead of WP:Foo, but was not redirected: nothing shows up under the page title as usual.
• Thảo luận:WP: ones: You ran into a warning that says "The requested page title refers to a talk page that cannot exist."

You also can't move, protect, or delete them, even though they still exist. And not only exist, some of them, such as WP:AutoWikiBrowser, are even defined as redirects; their name appear in italic. [[https://vi.wikipedia.org/wiki/Special:PrefixIndex/Wikipedia:AutoWikiBrowser|Wikipedia:AutoWikiBrowser]] also exist, but is not a redirect.

What should have happened instead?:
The old WP: and Thảo luận:WP: pages should be automatically moved to Wikipedia:s and Thảo luận Wikipedia:, accordingly. Needless to say, MediaWiki should also know what to do when the target page has already been created.

I asked @Urbanecm at T293839: Set default namespace aliases for projects, but he did not respond. Most of WP: pages were moved thanks to @Majavah, however.