Interfaces connected by bridge(4) do not pass arp replies

With 2 interfaces present on a bridge0, an arp request is send from
msk0 and is received
by a machine on em0.

When a reply to that arp request is sent by a machine on em0 it is not
visible on the
bridge0 nor on msk0 as indicated by tcpdump.

The arp reply is visible while watching em0 with tcpdump.

This behaviour is also true when arp requests are sent from a wlan
interface ath0
and received on a wired interface, the arp reply is visible on the
wired interface but not
visible monitoring with 'tcpdump -i bridge0 arp'.

The machine running the em0, msk0 and ath0 interfaces and bridge0 can
receive arp requests
from any interface and the arp replies are received fine from the
machine by the arp requester -err requestor.

Any help is greatly appreciated.

thanks
-kim