Are you the user in the proofreading group and are you the user who performs the transition?
Then the notification is not sent.

Thank you for pointing this out. Now it works. :)
But I wonder if this behavior is useful. First, the configuration of Notification Mail recipients refers to a freely definable "user group" and there is no indication that someone belonging to this group is excluded from the filter. Moreover, it means that a user can never get the notifications for himself.

Same as in forums software or here on github. I get notifications for every post in this repo - but never for mine, seems to be the usual behaviour in notification systems.
Would some more information in the message help?

Yes, I guess that's an argument. However, one could counter it with the fact that there are quite different types of notifications: Those that inform about an utterance/response in an ongoing conversation (and invite you to reply to a post) and those that only notify about a state or a state change.

The latter are notifications such as notifications about available updates, successfully performed tasks such as backups (Joomla tasks).

In the first case, the notification to the "author" / creator of a post, for example, is only usual, logical, expected in a particular case, since an action by the writer has already taken place. In the second case, however, it can be about general monitoring, just as I as an administrator can subscribe to system messages.

If you ask me, I would want notifications about a state change in a workflow in both cases: If you subscribe yourself as a user to the notification recipients, you should be allowed to receive these notification mails.

See also #37860