vMX: QinQ problems

I have the following topology:

R1--[ge-0/0/0]--R2--[ge-0/0/1]--R3--[ge-0/0/0]--R4

R1&R4 are user devices; they send tagged frames of VLAN12.

R2&R3 are performing QinQ; service VLAN ID is 100.

R2 configuration:

interfaces {
    ge-0/0/0 {
        unit 0 {
            family bridge {
                interface-mode trunk;   
                vlan-id-list 12;
            }
        }
    }
    ge-0/0/1 {
        flexible-vlan-tagging;
        unit 0 {
            vlan-id 100;
            family bridge {
                interface-mode trunk;
                inner-vlan-id-list 12;
            }
        }
    }
}
bridge-domains {                        
    VLAN_12 {
        vlan-id-list 12;
    }
}

R3 configuration: 

interfaces {
    ge-0/0/0 {
        flexible-vlan-tagging;
        encapsulation flexible-ethernet-services;
        unit 0 {                        
            encapsulation vlan-bridge;
            vlan-id-range 1-4094;
        }
    }
    ge-0/0/1 {
        flexible-vlan-tagging;
        encapsulation flexible-ethernet-services;
        unit 0 {
            encapsulation vlan-bridge;
            vlan-tags outer 100 inner-range 1-4094;
        }
    }
}
bridge-domains {
    VLAN_ALL {
        vlan-id all;                    
        interface ge-0/0/0.0;
        interface ge-0/0/1.0;
    }
}

The problem is R1 is unable to ping R4 and vice versa.

Traffic from R1 is able to make it way to R4 - I can tell it by MAC tables and by traffic capture.

Traffic from R4, however, makes its way up to R2's ge-0/0/1 and then is silently dropped.

R4's MAC adrress is not even learned by R2.