Currently, Active Roles Scheduled Tasks and Automation Workflows are linked to a specific Active Roles instance. If that instance is offline when the Scheduled Task or Automation Workflow is scheduled to fire, the operation is missed and there is no redundancy with an active Active Roles instance.
Real-time updates for Dynamic Groups are handled by any available Active Roles instance, however, bulk updates in excess of 1000 membership changes are handled by a process which is linked to a specific Active Roles instance, and also will not execute if that specific instance is offline.
This is currently by design.
An Enhancement Request (100498) has been created to add High Availability for these components.