NPF, bridge and IPv6

[triaxx <triaxx%triaxx.org@localhost>]

On NetBSD 8.0 with Xen, I experienced some strange behaviors with npf, 
bridged interfaces and IPv6.

The following schema represents a part of my network topology for which 
I encountered a problem:
|- dom0 -----------------------------------------------|
|                                 |- domU --------|    |
|-----|                 |---------|---------|     |    |
| wm0 |---- bridge0 ----| xvif1i0 | xennet0 |     |    |
|-----|                 |---------|---------|     |    |
|                                 |---------------|    |
|                                                      |
|                                 |- domU---------|    |
|    |------|           |-------- |---------|     |    |
|    | tap0 |- bridge1 -| xvif2i0 | xennet0 |     |    |
|    |------|           |---------|---------|     |    |
|                                 |---------------|    |
|------------------------------------------------------|

In my /etc/npf.conf, I have the following rules:
% group "external" on wm0 {
%   pass stateful out final all
%   pass stateful in final family inet4 proto tcp to wm0 port ssh
%   pass stateful in final family inet6 proto tcp to wm0 port ssh
%   [...]
% }
I understood that 'pass stateful out final all' allowed all outgoing 
connections and established incoming connections.

The imaps (port 993) is not allowed in my 'pass in' rules.

If I comment the line 'pass stateful out final all' and I try to connect 
from my dom0 to a remote host on port 993, the connection is refused:
dom0# telnet -4 trashware.triaxx.org 993
Trying 88.180.169.84...
telnet: Unable to connect to remote host: Network is unreachable

Same behavior using IPv6:
dom0# telnet -6 trashware.triaxx.org 993
Trying 2a01:e35:8b4a:9540:7271:bcff:fe94:3759...
telnet: Unable to connect to remote host: Network is unreachable

If I try to connect from the first domU (linked to bridge0), it 
surprisingly works using IPv4:
dumU# telnet -4 trashware.triaxx.org 993
Trying 88.180.169.84...
Connected to trashware.triaxx.org.
Escape character is '^]'

But it doesn't work using IPv6, even if I uncomment the line 'pass 
stateful out final all':
domU# telnet -6 trashware.triaxx.org 993
Trying 2a01:e35:8b4a:9540:7271:bcff:fe94:3759...

It's like if stateful was broken for IPv6...

Following is the reminder on what works and what doesn't:
                              domO/IPv4  dom0/IPv6  domU/IPv4  domU/IPv6
 pass stateful out final all      X          X          X          O
#pass stateful out final all      O          0          X          O

Is there known issue related to my experience?