With below existing config for gNMI, the gNMI requests are not being responded by MX, while fxp is in mgmt_junos

set system services extension-service request-response grpc clear-text port 57400

set system services extension-service request-response grpc max-connections 4

From server where we run gnmi query:

root@server:~# gnmic -a abc:57400 -u root -p "xyz" --insecure sub -e proto --mode once --path '/interfaces/interface/state/admin-status'

^[^C

received signal 'interrupt'. terminating...

In MX monitor traffic, we can see only SYN coming in & no SYN-ACK

In MX traceoptions:

Jun 10 09:46:59 ProcessInPendingCall: Processing pending call for RPC: /telemetry.OpenConfigTelemetry/telemetrySubscribe with tag id: request_2

The reason is, since you have management-instance, the extension service should be available under same instance.

MX will always respond back in mgmt_junos instance when replying back on the query & when gnmi config is not added under this instance, they will never reach fxp.

Adding below config resolved the issue. 

set system services extension-service request-response grpc routing-instance mgmt_junos