Microsoft Bookings double booking

The bookings system I have created is double-booking appointments for one of my staff; in that it's not reading Outlook events properly, nor is it recognising scheduled 'Time off' that has been manually added into the bookings calendar.
The staff member continues to be 'available' for appointments despite in staff view showing that they've got prebooked time away (see attached).

All the staff member's details are correct and current, and the checkbox 'Events on Office calendar affect availability' is checked too.

It may be a syncing issue but thought I get any other workarounds/ideas or see if anyone knows a fix.