Message list leaks information about thread participants when sending to role

Apologies if this is a duplicate, I couldn't find any mention of it in the open issues list.

Steps to repeat:
1. Create two groups of users, e.g. admins and authenticated users.
2. Grant authenticated users permission to send messages to other authenticated users, with a limit of one recipient per message. Do not grant any permissions to send to users by role.
3. Grant admins the additional permission to send a message to role {authenticated users}.
4. Send message from admin A to role {authenticated users}.
5. Send reply from authenticated user B to message from A.

Expected results:
Other authenticated users are unable to tell that B sent a reply to A.

Actual results:
Every authenticated user sees B's username in the list of thread participants in their inbox.

Notes:
1. I am aware of the Bulk Messages add-on module that allows one-way sending to all users, but this doesn't meet our needs. We would like admins to be able to make announcements to all users and still be able to receive private responses to the announcements.
2. Further to this, if another admin C sends a reply to the message, this will automatically be copied to role {authenticated users}, much like "Reply to All" in email. This may not be the desired behaviour, the option to restrict the reply to original sender (A) only would be very useful.