This issue happens when family inet/inet6 and vlan-bridge are configured on the same ifd in flexible-ethernet-services and VSTP runs on 

the vlan on vlan-bridge.

In this example, vlan11 and vlan792 are assigned to et-0/0/48 and vlan11 is encapsulation vlan-bridge and vlan792 is family inet/inet6.

VSTP is configured on vlan11 and *NOT* configured on vlan792. In this case, et-0/0/48 cannot recieve any packets on vlan792.

root@QFX5K> show configuration | display set | match et-0/0/48

set interfaces et-0/0/48 flexible-vlan-tagging
set interfaces et-0/0/48 mtu 9216
set interfaces et-0/0/48 encapsulation flexible-ethernet-services
set interfaces et-0/0/48 unit 11 encapsulation vlan-bridge
set interfaces et-0/0/48 unit 11 vlan-id 11
set interfaces et-0/0/48 unit 792 vlan-id 792
set interfaces et-0/0/48 unit 792 family inet address 1.1.1.1/24
set protocols vstp interface et-0/0/48
set protocols vstp interface et-0/0/48
set protocols vstp vlan 11
set vlans vlan-11 interface et-0/0/48.11

This issue might be seen if the following conditions are met:
* On Junos QFX5K series and EX series which support Enhanced Layer 2 Software (ELS)
* Family inet/inet6 and vlan-bridge are configured on the same ifd in flexible-ethernet-services.
* VSTP runs on all vlans (vlan-bridge) configured on the interface in question.  In this example, vlan11.
* Immediately after a reboot.

When ping is initiated from QFX to the direct connected peer on vlan792, ping will be TIMEOUT and cannot resolve ARP. The root cause is that VSTP is configured on the interface and we have to take care of all vlans on the interface in STP Hardware table even though STP is not configured on the vlan for inet/inte6 interface. In this case, STP HW entry was not properly set on vlan792 and all packets were being dropped on vlan792.

root@QFX5K> ping 1.1.1.2
PING 1.1.1.2 (1.1.1.2): 56 data bytes
^C
--- 1.1.1.2 ping statistics ---
15 packets transmitted, 0 packets received, 100% packet loss
{master:0}
root@QFX5K>

This has been fixed by the fix of 1732718.
As a workaround, add one more vlan-bridge on the interface in question. In this example, the interface is et-0/0/48 and do not enable VSTP on the new vlan.
You can use any vlan-ID. Below is the example.

set vlans vlan-4000 interface et-0/0/48.4000 
set interfaces et-0/0/48 unit 4000 encapsulation vlan-bridge 
set interfaces et-0/0/48 unit 4000 vlan-id 4000