I found an external issue that after Redis restart, sometimes in ConnectionRestored event the Redis connection is not reconnected yet, however after a few seconds eventually connection become reconnected, see the external issue StackExchange/StackExchange.Redis#61.

So in the StackExchange Redis scale-out, it will be better to add a workaround to wait Redis Connection become real connected in ConnectionRestored event handler, then reading Redis Key /sending message can always work after that.
Otherwise when in ConnectionRestored event the Redis connection is not connected yet, reading Redis Key /sending message will throw RedisConnectionException.