Ok, after quite a while, I found out what's wrong and can give a bit more details about intercommunication.
From the Aruba Airgroup doc I found the following :
When an AirGroup controller has the list of all the controllers in the multi-controller table, it will set up a Layer-2 GRE tunnel with all the controllers in the table. The Layer-2 GRE tunnel carries AirGroup specific packets only.
By looking at my GRE tunnels table, I realized that the GRE tunnels are using the controller's virtual IP address, which makes sense :)
So I added the virtual IP in the Airgroup domain and removed the direct path to VLAN 315 and it's working.