Ok, lets forget the fact you are using R77.30, yes, we all know its unsupported and it has been for long time, but lets see if we can help you out. So, here is my thinking, logically...
So, if if tunnel is up, that tells us that both phase 1 and 2 are correct, for sure. Now, if you say this only happens with ikev2 and not ikev1, can you run quick vpn debug while generating traffic and gather ike files from $FWDIR/log directory on the firewall, as well as vpnd.elg
Just turn on debug by running vpn debug ikeon, generate some traffic, wait couple of minutes and run vpn debug ikeoff to turn debug off.
If you could email me the stuff directly with any relevant IP addresses, I can check it later to see what could be going on. I definitely remember having ikev2 tunnels work back in R77.30...not often, but it did work, for sure.