Service Appointment with pinned Status can be manually rescheduled

This is an expected behavior. Service Appointments with a Pinned Status (App Launcher > Field Service Settings > Scheduling > Select which statuses are considered as pinned, or unmovable, for scheduling) can be manually rescheduled. The pinned Status feature is designed to prevent rescheduling Service Appointments using scheduling actions (e.g. Book Appointment, Get Candidates).

On the Service Appointment record, if the "Pinned" field is set to true, then the scheduled start and scheduled end of that service appointment cannot be updated.