When a L2circuit is configured in a QFX5120 and EX4650 equipment. Router Advertisement/Router Solicitation will not send to the peer. 
End client is not able to get auto-configured IPV6 address with SLAAC enabled IPv6 RA forwarding through L2Circuit over MPLS.

Non-working path: with MPLS L2 circuit between PE1 and PE2.

CPE1(SRX)----PE1(qfx5120-48y-8c)---P(QFX5100)---PE2(QFX5110)---CPE2

This setup was working with PE1 being used as QFX5100. 
Customer replaced the QFX5100 with QFX5120 and since then has been facing issue.
Customer used same config from QFX5100 to QFX5120 and its not working.

1. l2 circuit works Ok and no issue with IPv4 traffic.
2. Only RA messages sent by SRX are being dropped by the QFX5120.
3. PCAP taken on egress of MPLS core interface towards router P - It does not have Router Advertisement flag going out as long as l2circuit is between CPE devices.
4. IPV6 auto configuration works fine for CPE if there is no l2-circuit in between CPEs.

When the issue happens IPv6 connectivity will be affected due to devices cannot automatically be configured. The issue can be checked by using the following command:

user@hostname> show ipv6 router-advertisement | match "Advertisements sent:|Interface

Limitation on QFX5120 and EX4650; this was resolved per an internal PR