Hi,
I believe that the connection remains persistant to the Secondary Endpoint server until the endpoint services are restarted (EDPA and WDP by default). I know that the endpoints pre v12 are not considered to be a fully "sticky" connection but even in the 11.x versions, the endpoint should remains on a known server regardless of primary connections until the services are restarted. (I believe)
You could always force the endpoint back to the primary server through the Enforce console but your quickest fix would be the restarting of the Endpoint services.
Hope that helps