Dual Tagged vlan are not working when going out of on AFT line card when compare with uKERN based line card.
Its a Day one issue on AFT based line cards.
It can be seen on MX/EX92K JUNOS platforms running AFT line cards MPC10E and above.

IXIA(9/1) <--(xe-0/1/3:2)mx304(xe-0/1/3:1)<---(xe-0/0/0:0)mx204 <-- mx204(xe-0/0/0:3.200) <--(11/1) IXIA

user@device> show l2circuit connections
Layer-2 Circuit Connections:
               
Neighbor: 192.168.0.100
    Interface                 Type  St     Time last up          # Up trans
    lt-0/0/0.6018(vc 300)     rmt   Up     Apr 26 16:51:44 2024           1
      Remote PE: 192.168.0.100, Negotiated control-word: No
      Incoming label: 16, Outgoing label: 20
      Negotiated PW status TLV: No
      Local interface: lt-0/0/0.6018, Status: Up, Encapsulation: VLAN
      Flow Label Transmit: No, Flow Label Receive: No
 
user@device> show interfaces xe-0/1/0 | match rate    
  Input rate     : 5072 bps (10 pps)
  Output rate    : 368 bps (0 pps)
 
user@device> show interfaces lt-0/0/0 | match rate
  Input rate     : 4472 bps (10 pps)
  Output rate    : 4472 bps (10 pps)
 
user@device> show configuration protocols l2circuit
neighbor 192.168.0.100 {
    interface lt-0/0/0.6018 {
        virtual-circuit-id 300;
        no-control-word;
        encapsulation-type ethernet-vlan;
        ignore-mtu-mismatch;
    }
}
 
user@device> show configuration routing-instances
test-200 {
    instance-type virtual-switch;
    bridge-domains {
        P2P-data {
            vlan-id 300;
            interface xe-0/1/1.300;
        }
    }
    interface lt-0/0/0.6019;
}
 
user@device> show interfaces xe-0/1/1 | match rate
  Input rate     : 0 bps (0 pps)
  Output rate    : 5112 bps (10 pps)