When an interface of a Link Aggregation (LAG) goes down in an Active/Active Clustering Deployment, the failover is not triggered.
When 2 interfaces are part of a LAG in Active/Active Clustering, they will be monitored as one interface. If one of them is down, SonicOS won't trigger the failover because the redundant interface is still UP.
This happens because when using Active/Active Clustering, the option Active/Standby Failover only when ALL aggregate links are down is enabled by default.
This behaviour is expected, however you may change it by disabling the option Active/Standby Failover only when ALL aggregate links are down from High Availability | Advanced.