channels: new connected channels may fail to receive message from group_send()

It looks occasionally some channels won’t receive messages from using group_send(), although they all successfully connected to our server. Disconnected and re-connect may resolve the issue. Here is the sample:

two clients connected via websocket and joined a same group partner_orders_1531

127.0.0.1:6379[1]> zrange asgi::group:partner_orders_1531 0 -1 withscores
1) "specific.mrOZIufu!GAaMMeISqdry"
2) "1549585254.4294083"
3) "specific.qUSiCEQD!TGlphabkczyd"
4) "1549585307.3211055"

then calling async_to_sync(channel_layer.group_send)

channel_layer = get_channel_layer()
async_to_sync(channel_layer.group_send)(partner_order_group_name, order_data)

but only one channel with channel name qUSiCEQD!TGlphabkczyd was able to receive the message and triggered

class PartnerOrderConsumer(WebsocketConsumer):
        ...
        def order_receive(self, event):
            self.send(text_data=event["text"])

related redis log :

Pip & Environment: Python 3.6.6 channels==2.1.5 channels-redis==2.3.1 daphne==2.2.3 Django==1.11.10 The server is hosted on Ubuntu 16.04.5 LTS with 2vCPUs/4GB MEMORY by Digitalocean. We have two daphne instances and using nginx for load balancing.