This issue aims these paragraphs from https://symfony.com/doc/current/mailer.html#always-send-to-the-same-address :

Use the allowed_recipients option to specify exceptions to the behavior defined
in the recipients option; allowing emails directed to these specific recipients
to maintain their original destination:

With this configuration, all emails will be sent to youremail@example.com,
except for those sent to internal@example.com, internal-monitoring@example.fr,
etc., which will receive emails as usual.

The wording "exceptions" and "except" is a bit misleading, to me it means that the mail is sent to allowed_recipients addresses and not recipients addresses anymore. But in fact, the mail is still sent to recipients addresses.

It's not a feature bug as the contributor intended to replicate the whitelist behaviour of swiftmailer, but it's explicitly mentioned in swiftmailer documentation that the mail is still sent to recipients:

all email addresses matching these regexes will be delivered, like normal, as well as being sent to dev@example.com

Linked issues : #19769 & #19775