CARP will not relinquish master state on one interface

[Freddie Cash]

Two systems running identical hardware and software (one system actually
rsync'd from the other).  Running 10-CURRENT just after the new CARP
implementation went in:

FreeBSD nexus2.sd73.bc.ca 10.0-CURRENT FreeBSD 10.0-CURRENT #1 r245195: Thu
Jan 10 10:29:16 PST 2013     root at nexus2.sd73.bc.ca:/usr/obj/usr/src/sys/NEXUS
i386

Each box has 2 interfaces configured:
  em0 is part of an untagged vlan
  em1 is part of 8 tagged vlans

The tagged vlan interfaces work correctly on both boxes, and CARP switches
back and forth between MASTER and BACKUP correctly, whether it be via
unplugging the network cable or via "ifconfig ... state" changes.

The untagged vlan on both boxes will not relinquish MASTER status.  If both
boxes are up, both em0 interfaces are up, then both boxes configure the
vhid as MASTER and nasty things happen to our traffic.

The network configuration is:

[remote site]------fibre link--------[fibre switch]----------[carp
box1]--------[internet]
[remote site]------fibre link----------/            \------------[carp box
2]-------/
[remote site]------fibre link---------/

The fibre links to the fibre switch are on untagged vlans; the link from
the switch to each carp box is a tagged vlan trunk.  And the link from each
carp box to the Internet router is an untagged vlan.

If only box1 is online, everything works correctly.

If only box2 is online, everything works correctly.

If both boxes are online, everything on em1 works correctly, and em0 shows
MASTER on both boxes.

If both boxes are online but em0 is down on 1 (either) box, everything
works correctly.

Running "tcpdump -n -i em0 -T carp | grep CARP" on both boxes shows the
CARPv2 traffic from both boxes, with the correct vhid, advbase, advskew for
each box.  But the logs on box2 show "master down".

I'm at a loss as to what to try next.  Everything works for all the vlan
interfaces on em1.  But nothing I've tried works for em0.  Within 2 seconds
of the link showing UP, it becomes MASTER.  On both boxes.

-- 
Freddie Cash
fjwcash at gmail.com