Strange L2 failure

Has anyone seen issues with IOS where certain MACs fail? 54:52:00 (kvm) fails out an old 10mbit port on a 7206 running 12.2 SRE. I've never seen anything like this. DHCP worked, ARP worked, and arp debugging showed responses for arp to the MAC, however, tcpdump on the host system showed no unicast or arp responses coming from the router, while the switch management ip and other stuff on the local segment communicated fine with the vm. This broke for IPv6 as well. I changed the vm's MAC to 54:51:00, 50:52:00 and still failed. Changed it to 40:52:00 and it works for both v4 and v6. Was there a change which would cause certain hardware to not accept a MAC starting with 50: or higher? Jack