Anomolies with [active_members $pool] and [LB::server addr]

You may simply be seeing a discrepancy between the actual load-balancing and the monitors. The monitors generally only run every 15 or 30 seconds, so it may actually take that much time before they notice a node down. This would potentially explain why you could occasionally be seeing LB_FAILED events as yes that event is triggered either because there are no active nodes or because the connection attempt to the server failed. However, the active_members command is driven by the status as reported by the monitors. Is it possible the monitors are flapping your nodes up and down very quickly? I'm not sure how you are determining the nodes are reported up by the monitors. If it merely through the GUI, then it's certainly possibly the node went down and right back up. You may want to check the logs.