MS Bookings ignoring existing appointments / calendar availability

I've been trying to resolve this issue for one of our Student Support departments all day and can't seem to find a solution.

At first, their booking calendar was ignoring the existing calendar appointments of all staff members except one.  So, we took the following trouble-shooting steps:

• Unchecked the 'Always show time slots in business time zone' box on in the 'Bookings Page' options, as I understand it's buggy.
• Had each member of staff select 'Can view when I'm busy' from their sharing permissions for 'People in my organisation'.
• Ensured all staff members have the same settings in MS Bookings, including the 'Events of Office calendar affect availability.'

Following this, when inspecting the staff overview in MS Bookings, I was able to see all appointments as 'busy' in the MS Bookings Calendar area, however two staff remain bookable from the booking page when they have existing calendar entries in place.  There is no discernable reason for this.

Is there something we're missing?  We have to open appointments for learners for the next Academic session, but two members of staff would have their annual leave and lunch breaks and any other appointments within the college ignored.