Emails not sent as BCC although selected

On the Mass Contact form settings page, I unchecked "Allow sender to override BCC setting" and checked "Send as BCC (hide recipents) by default".

When creating a Mass Contact, the checkbox for "Send as BCC (hide recipients)" was checked and greyed out, ie the user could not change the setting.

This caused all emails to be sent in the TO field (not as BCC as selected).