IPv6 over IPv4 tunnel are brought up with TTL set to "inherit". This means IPv4 packet will have TTL of encapsulated packet. In the case of MLD and PIM packets with a TTL of 1: the TTL will be set to 0 at first IPv4 router encountered and message will not reach the other end of the tunnel.

To solve this behavior just fixe the TTL of the tunnel with something like:

ip tunnel change sit1 ttl 200