I would like to know why not downlink immediately. I attach picture below.
Because that is how the LoRaWAN protocol works. Please refer the LoRaWAN specification for more information: https://lora-alliance.org/lorawan-for-developers (look for: receive windows).
I test follow you tell me. I see message of MQTTT but I would like to know downlink why not immediately.
As said, LoRaWAN (Class-A and Class-B) does not provide functionality where you can send messages immediately to a device. They have to be scheduled to match the RX1 or RX2 receive-window. That is why immediately is false and the timestamp is set. This is all documented, please refer to the LoRaWAN specification.
Thank you for your answer. I found a problem. My gateway can not use downlink.