We use load balancing with balancing rules routing SIP traffic to a preferred gateway. When it fails a new one is chosen, but while UDP NAT sessions are alive on the new gateway, it is used regardless the status of the preferred one.
The only way to switch your gateway back is to let NAT sessions expire (turn SIP user agents off) or manually disable the new gateway. I guess it does not happen with http connections because their lifespan is shorter, while UDP ones could last forever.