this isn't an Aruba Instant question it seems, for controller based networks please use "Unified Wired & Wireless Access"
not 100% sure this is possible, but if it is then i would first double check the settings.
i assume you created the two internal users with the two different roles?
did you create your server group yourself or do you use the default one for the internal server? does it have the Server Rule to set the role?
for double checking it myself be wise to just put up all relevant config here, perhaps we spot something you missed.