Strange output of "mosquitto_sub -v -t "gateway/#""

ChirpStack Application Server
#1

I typed command mosquitto_sub -v -t "gateway/#" inside docker container eclipse-mosquitto from chirpstack deployed by docker-compose.

And it gives me this output:

/ # mosquitto_sub -v -t "gateway/#"
gateway/3dd8da8c6180df9b/state/conn
=�ڌa�ߛ►☺

Then I change output format to json and saw this:

/ # mosquitto_sub -d -v -t "gateway/#"
Client null sending CONNECT
Client null received CONNACK (0)
Client null sending SUBSCRIBE (Mid: 1, Topic: gateway/#, QoS: 0, Options: 0x00)
Client null received SUBACK
Subscribed (mid: 1): 0
Client null received PUBLISH (d0, q0, r1, m0, 'gateway/3dd8da8c6180df9b/state/conn', ... (45 bytes))
gateway/3dd8da8c6180df9b/state/conn {"gatewayID":"PdjajGGA35s=","state":"ONLINE"}
Client null received PUBLISH (d0, q0, r0, m0, 'gateway/3dd8da8c6180df9b/state/conn', ... (46 bytes))
gateway/3dd8da8c6180df9b/state/conn {"gatewayID":"PdjajGGA35s=","state":"OFFLINE"}
Client null sending PINGREQ
Client null received PINGRESP

How can I interpret this message?

P.S.: By the way, gateway detail Last seen at is always Never in Application Server UI.