Failover is not triggered even though an interface is physically monitored under High Availability | Monitoring: this happens when the interface is not configured but there are VLANs under this interface.
SonicOS does not monitor Unassigned Interfaces even if they're connected and monitored under High Availability | Monitoring.
If you want to monitor the status of a Physical Interface (i.e. X16), this interface must be configured: VLANs are not monitored for failover by the SonicWall.
Please, configure the interface (Network | Interfaces) with a zone and an IP Address so the SonicWall can monitor the physical interface for UP/DOWN activities and trigger the failover if the interface goes down:
NOTE: The opposite applies to configured interfaces when not monitored on High Availability | Monitoring: when the interface is configured, it will always be monitored no matter if Physical Monitoring is disabled on High Availability | Monitoring.