Q-in-Q transit traffic will be lost when Tag Protocol ID (TPID) is different than 0x8100

In particular MX series routers, when Q-in-Q tunneling is configured and the service provider vlan (S-Vlan) uses a Tag Protocol ID (TPID) different than 0x8100 such as 0x9200, 0x88a8 and 0x9100, transit traffic will not reach the next hop and will be lost. However, if the traffic is generated in the router itself, the tags are kept and the traffic will reach its destination. The platforms affected are MX204, MX240, MX304, MX480, MX960, MX2010, MX2020, MX10003, MX10004, MX10008 and MX10016.

For example, if the interface is configured with the outer vlan-tag 0x88a8:

user@router# show interfaces ae10.100
vlan-tags outer 0x88a8.xxx inner 0x8100.zzz;
family inet {
    address x.x.x.1/23 {
        vrrp-group <group-name> {
            virtual-address x.x.x.10;
            accept-data;
        }
    }
}

This problem is tracked and fixed via PR1841876:

https://prsearch.juniper.net/problemreport/PR1841876