I submitted a ticket:
"Hi. There is a bug in the new interface that allows recipients to reply to an entire contact group even when the mail is sent as 'Bcc'."
...and linked to this thread.
I just received this response:
"So, it appears this is a known issue and has been this way for years. It's to do with the initial design of alias targeting groups. I am afraid we are reluctant to make any immediate changes because there would be several users relying on the current behavior. We might address this in the future, but no timeline at the moment, sorry.
We actually have a separate service where all this is done rightly and for the specific use case we would recommend using that. Its called Topicbox: https://www.topicbox.com/
Let me know if you have any further questions."