A QinQ problem with QFX5100

Hello,

I have a mixed VC (QFX5100 and 3500) with 14.1X53-D40.8. There's an interface that incapsulates C-VLANs into S-VLAN:

set interfaces ae31 flexible-vlan-tagging 
set interfaces ae31 mtu 9216 
set interfaces ae31 encapsulation extended-vlan-bridge
set interfaces ae31 unit 3174 vlan-id-list 21-22
set interfaces ae31 unit 3174 input-vlan-map push
set interfaces ae31 unit 3174 input-vlan-map vlan-id 3174
set interfaces ae31 unit 3174 output-vlan-map pop

And on the other side the S-VLAN 3174 just goes out:

set interfaces ae0 flexible-vlan-tagging
set interfaces ae0 mtu 9216
set interfaces ae0 encapsulation extended-vlan-bridge
set interfaces ae0 unit 3174 vlan-id 3174

And S-VLAN configured like this:

set vlans sv3174-qinq interface ae0.3174
set vlans sv3174-qinq interface ae31.3174

At this point everything works fine, but if I create a VLAN with the same ID as a C-VLAN (21 or 22):

set vlans v21-user vlan-id 21

the traffic in the C-VLAN 21 stops.

What's wrong with it and how can I fix it?

Kind regards.