Guest staff

We have a Bookings that offers one service, attached to one staff. The staff is a Shared Mailbox. When we add the user to Staff, it initially looks like they are added as a team member, but after saving, it changes them to a Guest, and we are not afforded the opportunity to check the option "Events on Office calendar affect availability". Therefore, when we block off an entire day for the shared mailbox, it is still bookable based on the business hours, avoiding previously booked slots, but not taking into consideration that the entire day is not bookable.  Everything else works perfectly except for this. Does the staff member need to have a standard mailbox vs a shared mailbox?