Synchronous Instructions have a list of assigned students that are designated to be apart of that Synchronous Instruction. When a Zoom Meeting is attached to a Synchronous Instruction, the SIS will add/remove registrations for the Zoom Meeting as needed.
When updating a Synchronous Instruction with a new Zoom Meeting or attaching one, you will need to update the list of students in order for the SIS to update the registrants for the Zoom Meeting. Simply re-saving the students should suffice in updating the meeting.