Firewall issue with IPv6 multicast client-server

[Perfect]

Hi,
I have a strange observation. Program details is as follows:
Multicast server joined the multicast group (FF02::1). Client sends
packet to FF02::1, which are successfully received by multicast
server. However, when server sends response back to client (on unicast
address), client fails to receive. On further digging, i found that
client app is also required to to be added in exception list of
firewall on client machine. Earlier, i was only disabling sever app.

Strangely, i never required firewall to be disabled at client side in
case of pure unicast sockets.

Any reason why the two cases behave differently?

Thanks.