MarkLogic requires that the Security database only have 1 primary forest.  Having more than one primary forest for the Security database can cause issues.  The Security database should have a single primary forest, and one replica forest to support High Availability.

More details available in the knowledgebase article How many forests should my Security database have?

Refer to our documentation Configuring the Security and Auxiliary Databases to Use Failover Forests for more information on HA configuration.