We just ran into the same problem outlined in this post:
The only work-around that I was able to do was to disable the "User Virtual MAC Addresses" option on the Kemp. That option has the following text: "By default, the HA subsystem only switches IP addresses and uses gratuitous ARPs. Enabling this option [turning virtual mac address on] will force the MAC address to also be switched. This option is only needed if gratuitous ARPs are not allowed."
With the "virtual mac" option enabled, the VIP of one of the Kemp load balancers gets the multicast mac (like you have): 00-00-5e-00-05-01 Disabling this option, the VIP gets a unicast MAC: 00-10-f3-3e-12-32
But now HA failover takes a few mins while the ARP table expires.
I'd love to know what anyone else had done to get around this with PowerConnect 188.8.131.52 firmware and make it work like it did in 4.x firmware.