Default agent doesn't receive the email

New site, administrator "webmaster".
Created some tickets categories, all with "webmaster" as the manager for that category.
webmaster is also defined as the "default" categories administrator
Created some new tickets:
- The user who opened the ticket has received the notification email,
- webmaster has not received any notification email

Defined a new administrator "Giorgio"
Created a new ticket category "web site" with "Giorgio" as the manager.
Created a new ticket with category "web site"
- The user received the notification email
- Giorgio received the notification email

Changed the "web site" category administrator as "webmaster"
Created a new ticket with category "web site"
- The user received the notification email
- webmaster does not received the notification email