No longer reproducible with Portal 6.2.x GIT ID: 25c4ca3ad345929c90863ed10a49fd61b8042533
LPS-35569 the behavior after creating a new site was changed, so now the user is redirected to the Site Administration > Site Settings view so he can continue editing site options.
The success message is not displayed with this new behavior, because it should actually be displayed in the create new site form. Since the user is redirected to the newly created site, it's obvious the request has been successful. If there were any errors, they would be displayed in the create site form.
The same behavior has been extended to creating a new Site Template. The user is also redirected to the administration view of the created site template. See