Hi Damion,
As far as I understand, you are adding two Directory providers in a single LiveCycle domain. One of the directory provider (DC2 and which has a 15 minute delay) overrides the user/groups added by the other. Currently, there is no support in LiveCycle to add a failover domain controller. You can contact Adobe support and they help you file a feature request.
Temporarily, you may delete DC2 directory provider from the domain to prevent this but that would not give you any failover advantage.
Thanks,
Neerav