In rare cases when gPRC is used on a Junos device, the system id field may be empty in the messages communicated to the server.
Issue the below command from junos CLI operational mode:

device-name> request pfe execute target fpc0 command "show agent sensors verbose":
 

ID: 539528113 Type: 7 Name: (sensor_1002)

Sensor Data: 0(0) bytes

---------------------------

ID: 539528114 Type: 4 Name: (sensor_1001)

Sensor Data: 4184(4156) bytes

 Jvision Header: system: , slot: 0, time: Thu Jul 13 11:35:42.579, sequence_number: 3170134, sensor_name: sensor_1001:/junos/system/cmerror/configuration/:/junos/system/cmerror/configuration/:PFE, version: 1.0
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We notice 2 things in this output:
First that the "system" field is empty/null
Second that the FPC hostname is null

The FPC hostname is usually inherited from Junos configuration at the first boot, then remains the same even after a system hostname configuration is changed, to solve/confirm this, we would need to try changing the hostname from junos cli, if that didn't help, then we would need to enforce the hostname down to the FPC by restarting the chassisd or otherwise reboot the box.

Upon solving the FPC hostname issue, the system field should be populated again and will follow the FPC hostname which would solve the original problem.