Cisco WAN :: 1921 / Multicast Not Received Properly Via Bridged Ethernet Interface

Using a small network (WAN) with three 1921 routers (IOS 15.1(T)) connected via E1 links. One host (industrial PC - core 2 due running Win XP Pro) connected to each router. The spare Gigabit Ethernet port on each router is bridged with the active one (so a portable management PC (laptop) can be plugged in there and communicate with the router and the industriual PC.
 
Multicast routing strategy between the routers is as per "Anycast - Static RP", with PIM sparse-mode enabled on all interfaces and sink RP defined on all three routers.
 
Problem occurs when a (IPV4) multicast application on one PC communicates with the others:
 
(a) IGMP V2 membersip reports etc. work correctly at the sender and at the other two PCs receiving the multicast stream

(b) Multicast routing on the WAN is working correctly; running Wireshark on the receiving PCs shows that multicast data is received on the expected group.

(c) But, there is an error in the Ethernet packets

i. The first packet's Ethernet header contains the correct destination MAC address 01:00:5E:aa:bb:cc where aa:bb:cc match with the last three octets of the mulicast group's address, and that packet is received OK by the listening multicast application

ii. However, subsequent packets' Ethernet headers have the wrong MAC address 01:00:5E:00:00:00 (the last three octets are all zeroes and these packets are discarded by Windows on the receiving PC and not seen by the multicast application.
 
Problem is related to the presence of integrated routing & bridging; if I delete the bridge virtual interface,disable bridging and give the two Gigabit Ethernet ports their own IP addresses, the multicast reception works correctly; all received multicast packets have te expected value on the destination MAC address (matching the group address)I have used the same integrated routing/bridging configuration successfully on Cisco 2611 and 2811 routers and there was no such issue with the multicast packets.Have I overlooked some subtle aspect of configuration in the 1921 router or have I uncovered a bug...?
 
For reference - snippets from router configuration scripts
:
In the non-working configuration (with Integrated Routing & Bridging)
 interface GigabitEthernet0/0
no ip address
duplex auto
speed auto

[code]....
 
In the working configuration all the bridging is gone and the two Gigabit Ethernet interfaces have a very plain & simple configuration in different subnets
 
interface GigabitEthernet0/0
ip address 192.168.212.1 255.255.255.128
no ip directed-broadcast
ip pim sparse-mode
duplex auto

[code]....