The RDS host load balancing feature has certain constraints.
Anti-infinity rules can prevent an application from being placed on an RDS host, regardless of the reported load preference. For more information, see Configure an Anti-Affinity Rule for an Application Pool.
Load balancing affects new application sessions only. An RDS host that contains sessions in which a user has previously run an application is always reused for the same application. This behavior overrides reported load preferences and anti-affinity rules.
Applications are launched on an RDS host where a user already has an existing session, even if the RDS host reports a BLOCK load preference.
RDS session limits prevent application sessions from being created, regardless of the reported load preference.