Acctually I remember upgrading to Aruba OS 6.2 I think Aruba changed the behaviour of sourcing the DHCP packets to be the recipient interface of the DHCP discover. I remember it since it caused a large incident at one of my customers when upgrading and the solution was to implement correct routing for the DHCP server to find its way back to the client subnet (the recipient interface of the controller in this case).
So according to me it should source with your recipient interface. I haven't tested it since then so I can´t say for sure, perhaps it´s been changed again.
Which version are you running?
Cheers,