The function simplenews_generate_url does not work for unsaved subscribers because those entities do not have an ID. The ID is needed as parameter for the route.
Try to send a newsletter test with a [simplenews-subscriber:manage-url] token to an email which does not already exist as subscriber.
You will get the following error: Symfony\Component\Routing\Exception\InvalidParameterException: Parameter "snid" for route "simplenews.subscriptions_manage" must match "[^/]++" ("" given) to generate a corresponding URL. in Drupal\Core\Routing\UrlGenerator->doGenerate() (line 209 of core/lib/Drupal/Core/Routing/UrlGenerator.php).
Check the subscriber ID for the tokens [simplenews-subscriber:confirm-url] and [simplenews-subscriber:manage-url] just as it is done for [simplenews-subscriber:unsubscribe-url].
Active
4.0
Code
I forgot to put this issue back into "needs review", please take a look at my response in the MR.