When you try to add an alias to a page that is not publied (validated), the selected page is not stored.
There is no alert so it looks like it does not work.

I suggest few solutions :

• display an alert to explain that the page is not set because it is not validated.
• store the page then there is two solution, the alis is available but is redirecting to a page that is not validated, or the alias is not working until the page is published.