Re: Q-in-Q - Unable to send multiple inner tags through single outer VLAN on MX

Try this:

ge-2/1/2 {
 description "--- UPLINK TO TNDM-METR-SW1 G1/5 NNI-TRAFFIC ---";
 flexible-vlan-tagging;
 mtu 9000;
 encapsulation flexible-ethernet-services;
 unit 1976 {
 encapsulation vlan-bridge;
 vlan-id 1976;
 family bridge {
 interface-mode trunk;
 inner-vlan-id-list [ 345 456 ];

It looks like you forgot to set encapsulation for unit 1976: