On QFX series switch which is working on EVO, IPv6 traffic forwarding failure could be seen with prefix /56. There is a software issue which is specific to that prefix, this KB describes the details.

IPv6 traffic failure could be seen when passing through QFX EVO platform. The issue is seen with prefix /56 only.
For example, QFX has IPv6 route with prefix /56.

user@Switch# run show route 2001:XXX:XX1:XXXX::

inet6.0: 11 destinations, 11 routes (11 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

2001:XXX:XX1:XXXX::/56
                   *[BGP/170] 00:00:53, MED 100, localpref 100
                      AS path: 65000 I, validation-state: unverified
                    >  to 2001:XXX:XXX2:XXXX::2 via ae1.0

However, fails to ping to that route though destination device is available.

user@Switch# run ping 2001:XXX:XX1:XXXX::1
PING 2001:XXX:XX1:XXXX::1(2001:XXX:XX1:XXXX::1) 56 data bytes
^C
--- 2001:XXX:XX1:XXXX::1 ping statistics ---
4 packets transmitted, 0 received, 100% packet loss, time 3085ms