This article explains that population of the Infected Hosts list may not work when the secondary SRX device has free subscription, and clarifies that both SRX devices must have the appropriate license subscription for the functionality to work properly.

When an SRX HA pair is enrolled into Sky ATP, each SRX device must be licensed for the functionality desired. If one SRX device does not contain a paid subscription, the HA Pair will default to Free Subscription. See Juniper Sky Advanced Threat Prevention License Types for more details.

Ensure that both SRX devices are entitled with a paid Sky ATP subscription to ensure proper functionality, in this case, population of Infected Hosts list when attempting to access the C&C Server.