Cannot change the sort order of a page tree

Description

Steps to reproduce:

1. Create a page & a sub page under it.
2. Change the order of the sub pages.
3. A dialog displays:

   Moving the page in the structure will change the sort order of page "[TN Page 1]" to "According to sort index".
   Are you sure you want to move the page?

4. Click the Yes button.

Expected: The sub page is re-ordered with new position.
Actual: No change. The order is as before.