The anti-affinity feature has certain constraints.
Anti-affinity rules affect 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.
Aniti-affinity rules do not affect application launches from within an RDS desktop session.
RDS session limits prevent application sessions from being created, regardless of anti-affinity rules.
In certain circumstances, the instances of applications on the RDS host might not be restricted to the maximum count that you specify. For example, View cannot determine the exact instance count if other applications for other pending sessions are in the process of being launched.
Inter-application anti-affinity rules are not supported. For example, large application classes, such as Autocad and Visual Studio instances, cannot be counted in a single rule.
Do not use anti-affinity rules in environments where end-users use Horizon Client on mobile clients. Anti-affinity rules can result in multiple sessions in the same farm for an end user. Reconnecting to multiple sessions on mobile clients can result in indeterminate behavior.